leetcode Permutation

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

求全排列，相当于回溯法中的排列树。

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        HashSet<List<Integer>> res = new HashSet<List<Integer>>();
         backtrace(0,nums.length,nums,res);
         return new ArrayList(res);
    }
    public void backtrace(int t,int n,int[] nums,HashSet<List<Integer>> res){
        if(t>=n){
            List<Integer> list = new ArrayList<Integer>();
            for(int i = 0;i<n;i++)
                list.add(nums[i]);
            res.add(list);                
        }
        else{
            for(int i = t ;i<n; i++){
                swap(nums,i,t);
                backtrace(t+1, n, nums, res);
                swap(nums,t,i);
            }
        }
    }
    
    public  int[] swap(int[] ints, int x, int y) {
        
        int temp = ints[x];  
        ints[x] = ints[y];  
        ints[y] = temp;  
        return ints;  
    }  
}