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package problems;

import java.util.*;

public class LeetCode47 {

public static List<List<Integer>> permuteUnique(int[] nums) {

int length=nums.length;

List<List<Integer>> ans=new ArrayList<>();

if(length==0){

return ans;

}

// 排序（升序或者降序都可以），排序是剪枝的前提

Arrays.sort(nums);

Deque<Integer> path = new ArrayDeque<>();

boolean used[]=new boolean[length];

dfs(nums,length,0,path,used,ans);

return ans;

}

// 深搜

private static void dfs(int[] nums, int length, int depth, Deque<Integer> path, boolean[] used, List<List<Integer>> ans) {

if(depth==length){

ans.add(new ArrayList<>(path));

return;

}

for(int i=0;i<length;i++){

if(used[i]){

continue;

}

// 剪枝条件：i>0是为了保证nums[i-1]有意义

// 写used[i-1]==false是因为nums[i-1]在深度优先遍历的过程中刚刚被撤销选择

if(i>0&&nums[i]==nums[i-1]&& !used[i - 1]){

continue;

}

path.addLast(nums[i]);

used[i]=true;

dfs(nums,length,depth+1,path,used,ans);

path.removeLast();

used[i]=false;

}

}

public static void main(String[] args) {

permuteUnique(new int[]{1, 1, 2});

}

}