leetcode------Permutations II ★★★★★★★★★不会

标题：	Permutations II
通过率：	25.7%
难度：	难

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

看别人的也没有看懂什么意思

public class Solution {
    public ArrayList<ArrayList<Integer>> permuteUnique(int[] num) {
          ArrayList<ArrayList<Integer>> res=new ArrayList<ArrayList<Integer>>();
        dfs(res,num,0);
        return res;
        
    }
    public void dfs(ArrayList<ArrayList<Integer>> res,int[] num,int start){
        if(start==num.length){
            ArrayList<Integer> tmp=new ArrayList<Integer>();
            for(int i=0;i<num.length;i++){
                tmp.add(num[i]);
            }
                res.add(tmp);
        }
        for(int j=start;j<num.length;j++){
            if(cons(num,start,j)){
            swap(num,j,start);
            dfs(res,num,start+1);
            swap(num,j,start);
            }
        }
    }
    public void swap(int []num,int a,int b){
        if(num[a]!=num[b]){
        int tmp=num[a];
        num[a]=num[b];
        num[b]=tmp;
        }
    }
    public boolean cons(int [] num,int start,int end){
        for(int i=start;i<end;i++){
            if(num[i]==num[end])return false;
        }
        return true;
    }
}