LeetCode 1487. Making File Names Unique【字符串,哈希表】中等

本文属于「征服LeetCode」系列文章之一，这一系列正式开始于2021/08/12。由于LeetCode上部分题目有锁，本系列将至少持续到刷完所有无锁题之日为止；由于LeetCode还在不断地创建新题，本系列的终止日期可能是永远。在这一系列刷题文章中，我不仅会讲解多种解题思路及其优化，还会用多种编程语言实现题解，涉及到通用解法时更将归纳总结出相应的算法模板。

为了方便在PC上运行调试、分享代码文件，我还建立了相关的仓库。在这一仓库中，你不仅可以看到LeetCode原题链接、题解代码、题解文章链接、同类题目归纳、通用解法总结等，还可以看到原题出现频率和相关企业等重要信息。如果有其他优选题解，还可以一同分享给他人。

由于本系列文章的内容随时可能发生更新变动，欢迎关注和收藏征服LeetCode系列文章目录一文以作备忘。

Given an array of strings names of size n. You will create n folders in your file system such that, at the ith minute, you will create a folder with the name names[i].

Since two files cannot have the same name, if you enter a folder name that was previously used, the system will have a suffix addition to its name in the form of (k), where, k is the smallest positive integer such that the obtained name remains unique.

Return an array of strings of length n where ans[i] is the actual name the system will assign to the ith folder when you create it.

Example 1:

Input: names = ["pes","fifa","gta","pes(2019)"]
Output: ["pes","fifa","gta","pes(2019)"]
Explanation: Let's see how the file system creates folder names:
"pes" --> not assigned before, remains "pes"
"fifa" --> not assigned before, remains "fifa"
"gta" --> not assigned before, remains "gta"
"pes(2019)" --> not assigned before, remains "pes(2019)"

Example 2:

Input: names = ["gta","gta(1)","gta","avalon"]
Output: ["gta","gta(1)","gta(2)","avalon"]
Explanation: Let's see how the file system creates folder names:
"gta" --> not assigned before, remains "gta"
"gta(1)" --> not assigned before, remains "gta(1)"
"gta" --> the name is reserved, system adds (k), since "gta(1)" is also reserved, systems put k = 2. it becomes "gta(2)"
"avalon" --> not assigned before, remains "avalon"

Example 3:

Input: names = ["onepiece","onepiece(1)","onepiece(2)","onepiece(3)","onepiece"]
Output: ["onepiece","onepiece(1)","onepiece(2)","onepiece(3)","onepiece(4)"]
Explanation: When the last folder is created, the smallest positive valid k is 4, and it becomes "onepiece(4)".

Constraints:

• 1 <= names.length <= 5 * 104
• 1 <= names[i].length <= 20
• names[i] consists of lowercase English letters, digits, and/or round brackets.

题意：给定一个长度为 n 的字符串数组 names ，你在你的文件系统中依次创建文件夹 names[i] 。由于两个文件不能同名，如果你输入了一个已经存在的名字，系统会给该名字后面加上一个后缀 (k) ，k 是使文件名字唯一的最小正整数。

解法 模拟+哈希表+字符串

一开始想复杂了，现在连想得多么复杂都不愿想起来了。总之，这个题目就是模拟文件系统的命名机制，给出一个文件名 names[i] ，如果没有出现过，则直接命名为 names[i] ；如果出现过，则从 (1) 开始给 names[i] 添加后缀 (k) ，直到找到没有出现过的文件名 "names[i](k)" ，这的 k 就是使文件名字唯一的最小正整数。

多说无益，看看几个示例：

• 对于 ["pes","fifa","gta","pes(2019)"] ，其中每个名字在之前都没出现过，所以输出就是 ["pes","fifa","gta","pes(2019)"] 。
• 对于 ["gta","gta(1)","gta","avalon"] ，"gta", "gta(1)" 在之前都没出现过，直接加入答案数组；后面的 "gta" 则在前面出现过，所以先加后缀命名为 "gta(1)" ，发现也出现过，于是命名为 "gta(2)" ，没有出现过，直接加入答案数组。输出就是 ["gta","gta(1)","gta(2)","avalon"] 。
• 对于 ["gta","gta(1)","gta","gta(2)"] ，前三个都是一样，到第四个 "gta(2)" 时，发现它出现过，和第三个 "gta" 重新命名后的名字相同，那就需要在 "gta(2)" 后面加后缀，先变为 "gta(2)(1)" ，发现没有出现过，就直接加入答案数组。

看了这几个示例，代码写起来也很简单了：使用哈希表 rec 存储所有第一次出现（包括「添加后缀之后首次出现」的字符串）的字符串、和后面的同名字符串将要使用的数字（一开始是1）。每次从 names 拿到一个新名字 names[i] 时：

1. 先判断 rec 中是否存在，若不存在，说明之前没出现过，可直接用作文件名，此时 rec[names[i]] = 1 ；
2. 否则，设 s = names[i] ，然后不断从 rec 中取出 names[i] 已经使用的数字 k 作为后缀拼在 s 后面，++rec[names[i] 。如果这一名字也已在 rec 中，就继续循环这一步，直到得到新的文件名，并设置 rec[s] = 1 。

• 时间复杂度：$O(n)$ 。如果都是不同字符串，则全部名字都可直接用，遍历一遍 names 数组即可；如果都是相同字符串，则要依次取 rec[names[i]] 、添加新的后缀、++rec[names[i]] 。
• 空间复杂度：$O(n)$

class Solution {public String[] getFolderNames(String[] names) {String[] ans = new String[names.length];var rec = new HashMap<String, Integer>();for (int i = 0; i < names.length; ++i) {String s = names[i];while (rec.containsKey(s) ) {int suffix = rec.get(names[i]);s = names[i] + "(" + suffix + ")";rec.put(names[i], suffix + 1);}rec.put(s, 1);ans[i] = s;}return ans;}
}