牛客题目链接

1. 题目考点

1. 回溯剪枝条件

2. 考点解析

1. 除开第一个重复元素外 其余重复元素开始全排必定重复

import java.util.*;
public class Solution {
    ArrayList<String> res = new ArrayList<>();
    public ArrayList<String> Permutation(String str) {
        if (str == null || str.equals("")) return res;
        char[] strs = str.toCharArray();
        // 关键：排序使重复元素相邻
        Arrays.sort(strs);
        boolean[] used = new boolean[strs.length];
        StringBuilder path = new StringBuilder();
        backTrace(strs,path,used);
        return res;
    }
    
    public void backTrace(char[] strs, StringBuilder path, boolean[] used) {
        if (path.length() == strs.length) {
            res.add(path.toString());
            return ;
        }
        
        for (int i = 0; i < strs.length; i++) {
            if (used[i] == true) continue;
            // 关键：剪枝条件 相邻元素相同 且前一个元素未使用
            if (i > 0 && strs[i] == strs[i-1] && used[i-1] == false)
                continue;
            used[i] = true;
            path.append(strs[i]);
            backTrace(strs, path, used);
            used[i] = false;
            path.deleteCharAt(path.length() - 1);
        }
    }
}