Permute.java

package com.leetcode.backtracking;

import java.util.ArrayList;
import java.util.List;

/**
 * @author: yhl
 * @DateTime: 2021/2/26 15:44
 * @Description:全排列
 */
public class Permute {

    public static void main(String[] args) {
        int[] nums = {1, 2, 3};
        Permute permute = new Permute();
        List<List<Integer>> result = permute.permute(nums);
        System.out.println(result);
    }

    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums.length == 0) {
            return res;
        }
        List<Integer> path = new ArrayList<>();
        int length = nums.length;
        boolean[] used = new boolean[length];
        dfs(nums, path, res, used);
        return res;
    }

    private void dfs(int[] nums, List<Integer> path, List<List<Integer>> res, boolean[] used) {

        if (path.size() == nums.length) {
            res.add(new ArrayList<>(path));
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (!used[i]) {
                path.add(nums[i]);
                used[i] = true;
                dfs(nums, path, res, used);
                // 注意：下面这两行代码发生 「回溯」，回溯发生在从 深层结点 回到 浅层结点 的过程，代码在形式上和递归之前是对称的
                used[i] = false;
                path.remove(path.size() - 1);
            }
        }
    }
}