Given a collection of distinct integers, return all possible permutations.

Example:

Input: [1,2,3]Output:[  [1,2,3],  [1,3,2],  [2,1,3],  [2,3,1],  [3,1,2],  [3,2,1]]

难度：medium

题目：给定一组集合元素，返回其所有排列数。

思路：递归

Runtime: 3 ms, faster than 70.98% of Java online submissions for Permutations.

class Solution {    public List
    
     
      > permute(int[] nums) {        List
      
       
        > result = new ArrayList<>();        permute(nums, 0, new Stack
        
         (), result);                return result;    }        private void permute(int[] nums, int idx, Stack
         
           stack, List
          
           
            > result) { if (idx == nums.length) { result.add(new ArrayList<>(stack)); return; } for (int i = idx; i < nums.length; i++) { stack.push(nums[i]); swap(nums, i, idx); permute(nums, idx + 1, stack, result); swap(nums, i, idx); stack.pop(); } } private void swap(int[] nums, int i, int j) { int t = nums[i]; nums[i] = nums[j]; nums[j] = t; }}