LeetCode 127. Word Ladder 广度优先搜索 BFS

Word Ladder - LeetCode
给定一个两个词 beginWord, endWord 与一个unordered_set wordList。从beginWord开始，每次更改一个字母变到wordList中包含的一个词。求最少几次可以从beginWord变到endWord

思路是从beginWord开始，查找在wordDict里面所有改变一个字母能变成的词，加到toVisit里面。然后在查找toVisit里面的词。这样一层一层的查找方法显然是BFS

当已经找到endWord时，查找已完成，返回res。PS：res初始为2(beginWord 与 endWord)

class Solution {
public:
    int ladderLength(string beginWord, string endWord, unordered_set<string>& wordList) {
        wordList.insert(endWord);
        queue<string> toVisit;
        bfs(beginWord, wordList, toVisit);
        int res = 2;
        while(not toVisit.empty()) {
            int size = toVisit.size();
            while (size--) {
                auto cur = toVisit.front();
                toVisit.pop();
                if (cur == endWord) return res;
                bfs(cur, wordList, toVisit);
            }
            res++;
        }
        return 0;
    }
private:
    void bfs(string word, unordered_set<string>& wordList, queue<string>& toVisit) {
        wordList.erase(word);
        for (int i = 0; i < word.length(); i++) {
            char originalLetter = word[i];
            for (int l = 0; l < 26; l++) {
                word[i] = 'a' + l;
                if (wordList.find(word) != wordList.end()) {
                    toVisit.push(word);
                    wordList.erase(word);
                }
            }
            word[i] = originalLetter;
        }
    }
};