720. Longest Word in Dictionary*-CFANZ编程社区

720. Longest Word in Dictionary*

https://leetcode.com/problems/longest-word-in-dictionary/

题目描述

Given a list of strings words representing an English Dictionary, find the longest word in words that can be built one character at a time by other words in words. If there is more than one possible answer, return the longest word with the smallest lexicographical order.

If there is no answer, return the empty string.
Example 1:

Input: 
words = ["w","wo","wor","worl", "world"]
Output: "world"
Explanation: 
The word "world" can be built one character at a time by "w", "wo", "wor", and "worl".

Example 2:

Input: 
words = ["a", "banana", "app", "appl", "ap", "apply", "apple"]
Output: "apple"
Explanation: 
Both "apply" and "apple" can be built from other words in the dictionary. However, "apple" is lexicographically smaller than "apply".

Note:

All the strings in the input will only contain lowercase letters.
The length ofwords will be in the range[1, 1000].
The length ofwords[i] will be in the range[1, 30].

C++ 实现 1

可以考虑先对字典进行排序, 规则是:

长度小的排前面
长度相等的, 但是字典序大的排前面

上述规则也可以倒过来, 比如(长度大的排前面, 同时长度相等的但字典序小的排前面).

然后从后向前遍历数组, 判断每个字符串是否能用其他字符串逐渐形成, 这一步使用哈希表完成. 从后向前, 当找到第一个满足条件的字符串, 直接返回即可.

class Solution {
private:
    struct Comp {
        bool operator()(const string &s1, const string &s2) {
            if (s1.size() < s2.size())
                return true;
            else if (s1.size() == s2.size())
                return (s1 > s2);
            return false;
        }
    };
public:
    string longestWord(vector<string>& words) {
        std::sort(words.begin(), words.end(), Comp());
        unordered_set<string> record(words.begin(), words.end());

        for (int i = words.size() - 1; i >= 0; --i) {
            bool found = true;
            for (int j = words[i].size() - 1; j > 0; j--)
                if (!record.count(words[i].substr(0, j))) {
                    found = false;
                    break;
                }
            if (found)
                return words[i];
        }
        return "";

    }
};

C++ 实现 2

速度更慢一些, 不排序, 而是对哈希表中的每一个字符串, 判断是否能用其他字符串逐渐形成, 同时还要判断是否满足题目中的条件.

class Solution {
public:
    string longestWord(vector<string>& words) {
        unordered_set<string> record(words.begin(), words.end());
        string res;
        for (auto &p : record) {
            if (p.size() >= res.size()) {
                bool contains = true;
                for (int i = 0; i < p.size(); ++ i) {
                    if (!record.count(p.substr(0, i + 1))) {
                        contains = false;
                        break;
                    }
                }
                if (contains) {
                    if (res.empty() || p.size() > res.size()) res = p;
                    else if (p.size() == res.size()) res = p < res ? p : res;
                } 
            }
        }
        return res;
    }
};