46. Permutations

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.
Memory Usage: 28 MB, less than 4.36% of Java online submissions for Permutations.

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        permute(nums, 0, new Stack<Integer>(), result);
        
        return result;
    }
    
    private void permute(int[] nums, int idx, Stack<Integer> stack, List<List<Integer>> 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;
    }
}