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‎guangxu/202205/20220514.md

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## Algorithm
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[46. Permutations](https://leetcode.com/problems/permutations/)
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### Description
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### Solution
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Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.
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Example 1:
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```
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Input: nums = [1,2,3]
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Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
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```
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Example 2:
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```
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Input: nums = [0,1]
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Output: [[0,1],[1,0]]
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```
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Example 3:
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```
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Input: nums = [1]
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Output: [[1]]
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```
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Constraints:
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```java
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- 1 <= nums.length <= 6
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- -10 <= nums[i] <= 10
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- All the integers of nums are unique.
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### Solution
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```java
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class Solution {
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public List<List<Integer>> permute(int[] nums) {
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List<List<Integer>> lists = new ArrayList<List<Integer>>();
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helper(nums, 0, lists);
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return lists;
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}
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private void helper(int[] nums, int start, List<List<Integer>> lists){
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if(start==nums.length){
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List<Integer> temp = new ArrayList<>();
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for(int num:nums){
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temp.add(num);
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}
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lists.add(temp);
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}else{
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for(int i=start;i<nums.length;i++){
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swap(nums, i, start);
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helper(nums, start+1, lists);
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swap(nums, i, start);
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}
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}
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}
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public static void swap(int[] nums, int start, int end){
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int temp = nums[start];
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nums[start] = nums[end];
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nums[end] = temp;
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}
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}
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```
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### Discuss
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Subsets : https://leetcode.com/problems/subsets/
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```Java
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public List<List<Integer>> subsets(int[] nums) {
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List<List<Integer>> list = new ArrayList<>();
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Arrays.sort(nums);
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backtrack(list, new ArrayList<>(), nums, 0);
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return list;
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}
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private void backtrack(List<List<Integer>> list , List<Integer> tempList, int [] nums, int start){
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list.add(new ArrayList<>(tempList));
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for(int i = start; i < nums.length; i++){
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tempList.add(nums[i]);
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backtrack(list, tempList, nums, i + 1);
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tempList.remove(tempList.size() - 1);
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}
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}
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```
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Subsets II (contains duplicates) : https://leetcode.com/problems/subsets-ii/
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```java
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public List<List<Integer>> subsetsWithDup(int[] nums) {
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List<List<Integer>> list = new ArrayList<>();
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Arrays.sort(nums);
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backtrack(list, new ArrayList<>(), nums, 0);
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return list;
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}
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private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, int start){
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list.add(new ArrayList<>(tempList));
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for(int i = start; i < nums.length; i++){
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if(i > start && nums[i] == nums[i-1]) continue; // skip duplicates
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tempList.add(nums[i]);
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backtrack(list, tempList, nums, i + 1);
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tempList.remove(tempList.size() - 1);
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}
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}
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```
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Permutations : https://leetcode.com/problems/permutations/
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```java
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public List<List<Integer>> permute(int[] nums) {
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List<List<Integer>> list = new ArrayList<>();
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// Arrays.sort(nums); // not necessary
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backtrack(list, new ArrayList<>(), nums);
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return list;
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}
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private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums){
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if(tempList.size() == nums.length){
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list.add(new ArrayList<>(tempList));
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} else{
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for(int i = 0; i < nums.length; i++){
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if(tempList.contains(nums[i])) continue; // element already exists, skip
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tempList.add(nums[i]);
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backtrack(list, tempList, nums);
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tempList.remove(tempList.size() - 1);
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}
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}
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}
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```
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Permutations II (contains duplicates) : https://leetcode.com/problems/permutations-ii/
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```java
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public List<List<Integer>> permuteUnique(int[] nums) {
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List<List<Integer>> list = new ArrayList<>();
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Arrays.sort(nums);
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backtrack(list, new ArrayList<>(), nums, new boolean[nums.length]);
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return list;
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}
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private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, boolean [] used){
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if(tempList.size() == nums.length){
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list.add(new ArrayList<>(tempList));
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} else{
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for(int i = 0; i < nums.length; i++){
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if(used[i] || i > 0 && nums[i] == nums[i-1] && !used[i - 1]) continue;
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used[i] = true;
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tempList.add(nums[i]);
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backtrack(list, tempList, nums, used);
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used[i] = false;
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tempList.remove(tempList.size() - 1);
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}
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}
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}
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```
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Combination Sum : https://leetcode.com/problems/combination-sum/
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```java
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public List<List<Integer>> combinationSum(int[] nums, int target) {
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List<List<Integer>> list = new ArrayList<>();
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Arrays.sort(nums);
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backtrack(list, new ArrayList<>(), nums, target, 0);
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return list;
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}
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private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, int remain, int start){
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if(remain < 0) return;
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else if(remain == 0) list.add(new ArrayList<>(tempList));
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else{
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for(int i = start; i < nums.length; i++){
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tempList.add(nums[i]);
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backtrack(list, tempList, nums, remain - nums[i], i); // not i + 1 because we can reuse same elements
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tempList.remove(tempList.size() - 1);
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}
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}
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}
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```
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Combination Sum II (can't reuse same element) : https://leetcode.com/problems/combination-sum-ii/
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```java
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public List<List<Integer>> combinationSum2(int[] nums, int target) {
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List<List<Integer>> list = new ArrayList<>();
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Arrays.sort(nums);
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backtrack(list, new ArrayList<>(), nums, target, 0);
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return list;
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}
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private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, int remain, int start){
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if(remain < 0) return;
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else if(remain == 0) list.add(new ArrayList<>(tempList));
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else{
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for(int i = start; i < nums.length; i++){
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if(i > start && nums[i] == nums[i-1]) continue; // skip duplicates
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tempList.add(nums[i]);
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backtrack(list, tempList, nums, remain - nums[i], i + 1);
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tempList.remove(tempList.size() - 1);
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}
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}
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}
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```
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Palindrome Partitioning : https://leetcode.com/problems/palindrome-partitioning/
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```java
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public List<List<String>> partition(String s) {
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List<List<String>> list = new ArrayList<>();
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backtrack(list, new ArrayList<>(), s, 0);
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return list;
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}
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public void backtrack(List<List<String>> list, List<String> tempList, String s, int start){
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if(start == s.length())
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list.add(new ArrayList<>(tempList));
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else{
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for(int i = start; i < s.length(); i++){
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if(isPalindrome(s, start, i)){
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tempList.add(s.substring(start, i + 1));
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backtrack(list, tempList, s, i + 1);
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tempList.remove(tempList.size() - 1);
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}
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}
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}
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}
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public boolean isPalindrome(String s, int low, int high){
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while(low < high)
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if(s.charAt(low++) != s.charAt(high--)) return false;
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return true;
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}
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```
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## Review
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‎guangxu/README.md

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| 标题 | Algorithm | Review | Tip | Share|
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| - | - | - | - | - |
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| [20220514](./202205/20220514.md) |[46. Permutations(全排列)](https://leetcode.com/problems/permutations/)||||
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| [20220513](./202205/20220513.md) |[322. Coin Change(兑换硬币-动态规划)](https://leetcode.com/problems/coin-change/)||||
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| [20220512](./202205/20220512.md) |[887. Super Egg Drop(扔鸡蛋问题-动态规划)](https://leetcode.com/problems/super-egg-drop/)||||
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| [20220511](./202205/20220511.md) |[773. Sliding Puzzle(滑动窗格-广度优先搜索)](https://leetcode.com/problems/sliding-puzzle/)||||

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