leetcode[127]Word Ladder

Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, 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) {
        unordered_set<string> add;
        add.insert(start);
        queue<string> q;
        q.push(start);
        int res=0;
        int levCurr=1,levNext=0;
        while(!q.empty())
        {
            string s0=q.front();
            q.pop();
            levCurr--;
            for(int i=0;i<s0.size();i++)
            {
                for(char j='a';j<='z';j++)
                {
                    if(s0[i]==char(j))continue;
                    string s=s0;
                    s[i]=char(j);
                    if(s==end)return res+2;
                    if(dict.find(s)!=dict.end()&&add.find(s)==add.end())
                    {
                        add.insert(s);
                        q.push(s);
                        levNext++;
                    }
                }
            }
            if(levCurr==0)
            {
                levCurr=levNext;
                levNext=0;
                res++;
            }
        }
        return 0;
    }
};