Word Ladder 分类： Leetcode（广度优先搜索） 2015-04-22 09:49 39人阅读 评论(0) 收藏

Word Ladder

Given two words (beginWord and endWord), and a dictionary, find the length of shortest transformation sequence from beginWord to endWord, such that:

1. Only one letter can be changed at a time
2. Each intermediate word must exist in the dictionary

For example,

Given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]

As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.

Note:

• Return 0 if there is no such transformation sequence.
• All words have the same length.
• All words contain only lowercase alphabetic characters.

class Solution {
public:
    int ladderLength(string start, string end, unordered_set<string> &dict) {
        if(start.size() != end.size()) return 0;
        if(start.empty() || end.empty()) return 0;
        
        queue<string> path;
        path.push(start);
        int level = 1;
        int count = 1;
        dict.erase(start);
        while(dict.size() >0 && !path.empty()) {
            string curword = path.front();
            path.pop();count--;
            for(int i = 0; i < curword.size(); i++) {
                string tmp = curword;
                for( char j = 'a'; j <= 'z';j++){
                    if(tmp[i] == j) continue;
                        tmp[i] =j;
                    if(tmp == end) return level+1;
                    if(dict.find(tmp) != dict.end()) path.push(tmp);
                    dict.erase(tmp);
                } 
            }
            
            if(count == 0) {
                count = path.size();
                level ++;
            }
        }
        
        return 0;
    }
};

 

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