46. Permutations

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]
]

解题思路为backtracking。mark已使用过的数字。

public class Solution {
    
    private boolean[] used;
    
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        used = new boolean[nums.length];
        helper(nums, new ArrayList<Integer>(), res);
        return res;
    }
    
    private void helper(int[] nums, List<Integer> list, List<List<Integer>> res) {
        if(list.size() == nums.length) {
            res.add(new ArrayList<Integer>(list));
            return;
        }
        for(int i = 0; i < nums.length; i++) {
            if(used[i]) continue;
            list.add(nums[i]);
            used[i] = true;
            helper(nums, list, res);
            used[i] = false;
            list.remove(list.size()-1);
        }
    }
}