leetcode 46. 全排列

题目

给定一个没有重复数字的序列，返回其所有可能的全排列。

示例:
输入: [1,2,3]
输出:
[
[1,2,3],
[1,3,2],
[2,1,3],
[2,3,1],
[3,1,2],
[3,2,1]
]

class Solution {
    public static List> permute(int[] nums) {
        List> res = new ArrayList<>();
        if (nums == null || nums.length == 0) return res;
        helper2(res, 0, nums);
        return res;
    }
    public static void helper2(List> res, int start, int[] nums) {
        if (start == nums.length) {
            List list = new ArrayList<>();
            for (int num : nums) {
                list.add(num);
            }
            res.add(new ArrayList<>(list));
            return;
        }
        for (int i = start; i < nums.length; i++) {
            swap(nums, start, i);
            helper2(res, start + 1, nums);
            swap(nums, start, i);
        }
    }
    public static void swap(int[] nums, int l, int r) {
        int temp = nums[l];
        nums[l] = nums[r];
        nums[r] = temp;
    }
}