LeetCode 46. Permutations（排列）

原题网址：https://leetcode.com/problems/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], and [3,2,1].

方法：深度优先搜索。

public class Solution {
    private List<List<Integer>> permutations = new ArrayList<>();
    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
    private void dfs(int[] nums, int step) {
        if (step == nums.length-1) {
            List<Integer> permutation = new ArrayList<>(nums.length);
            for(int i=0; i<nums.length; i++) permutation.add(nums[i]);
            permutations.add(permutation);
            return;
        }
        for(int i=step; i<nums.length; i++) {
            swap(nums, step, i);
            dfs(nums, step+1);
            swap(nums, step, i);
        }
    }
    public List<List<Integer>> permute(int[] nums) {
        dfs(nums, 0);
        return permutations;
    }
}