leetcode : permutation

Given a collection of distinct 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],
  [3,2,1]
]

思路： backtrack

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(nums == null || nums.length == 0) {
            return result;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        helper(result,list,nums);
        return result;
    }
    
    public void helper(List<List<Integer>> result, ArrayList<Integer> list, int[] nums) {
        
        if(list.size() == nums.length) {
            result.add(new ArrayList<Integer>(list));
        }
        
        for(int i = 0; i < nums.length; i++) {
            if(list.contains(nums[i])) {
                continue;
            }
            list.add(nums[i]);
            helper(result,list,nums);
            list.remove(list.size() - 1);
        }
    } 
}