题解 | #字符串的排列#

import java.util.*;
import java.io.*;
public class Solution {
    public ArrayList<String> Permutation(String str) {
        if (str == null) return null;
        char[] nums = str.toCharArray();
        ArrayList<String> list = new ArrayList<>();
        if (nums.length == 0) return list;
        dfs(0, nums, list);
        return list;
    }

    private void dfs(int idx, char[] nums, ArrayList<String> list) {
        // 不能再往下搜索
        if (idx == nums.length) {
            StringBuilder result = new StringBuilder();
            for (char value : nums) {
                result.append(value);
            }
            list.add(result.toString());
            return;
        }

        // 枚举这一层所有可以做出的选择
        Set<Character> set = new HashSet();
        for (int i = idx; i < nums.length; i++) {
            if(set.contains(nums[i])){
                continue;
            }
            set.add(nums[i]);
            swap(nums, idx, i);
            dfs(idx + 1, nums, list);
            swap(nums, idx, i);
        }
    }

    private void swap(char[] nums, int i, int j) {
        char tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

解题思想：DFS标准模块，主要是 枚举这一层所有可以做出的选择