Word Ladder

LeetCode – Word Ladder

 

Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that only one letter can be changed at a time and each intermediate word must exist in the dictionary. For example, given:

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

One shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog", the program should 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 == end){
            return 0;
        }
        unordered_map<string,int> len;
        len[start] = 1;
        queue<string> q;
        for(q.push(start);!q.empty();q.pop()){
            string word = q.front();
            int step = len[start] + 1;
            for(int i = 0;i<word.length();i++){
                for(char ch = 'a';ch <= 'z';++ch){
                    if(word[i] != ch){
                        char temp = word[i];
                        word[i] = ch;
                        if((dict.find(word)!=dict.end())&&(len.find(word)==len.end())){
                            if(start == end){
                                return step;
                            }
                            q.push(word);
                            len[word] = step;
                         }
                        word[i] = temp;
                    }
                }
            }
        }
        return 0;
    }
}