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;
}
}