Title

the problem link

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].

题目大意：求字典里最长的单词，这个单词可以由其他的一个一个字母的加上组成

Solution

//cpp:
class Solution {
public:
	string longestWord(vector<string>& words) {
		unordered_set<string> set1;
		for (auto it : words) {
			set1.insert(it);
		}
		string ans = "";
		for (int i = 0; i < words.size(); i++) {
			if (words[i].length() > ans.length() || words[i].length() == ans.length() && words[i] < ans) {
				bool flag = true;
				for (int k = 1; k < words[i].length(); k++) {
					if (set1.find(words[i].substr(0, k)) == set1.end()) {
						flag = false;
						break;
					}
				}
				if (flag) ans = words[i];
			}
		}
		return ans;
	}
};