[LeetCode] Longest Word In Dictionary

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 of words will be in the range [1, 1000].
• The length of words[i] will be in the range [1, 30].

输出字典序最小的最长字典字符串。

首先先对words排序，则字典字符串都在一起。然后对每一个字符串进行判断，保存尺寸最长的字典字符串。

class Solution {
public:
    string longestWord(vector<string>& words) {
        sort(words.begin(), words.end());
        unordered_set<string> ss;
        string res;
        for (string word : words) {
            if (word.size() == 1 || ss.count(word.substr(0, word.size() - 1))) {
                res = word.size() > res.size() ? word : res;
                ss.insert(word);
            }
        }
        return res;
    }
};
// 46 ms