[Leetcode] Word Ladder

class Solution {
public:
    int ladderLength(string start, string end, unordered_set<string> &dict) {        
        unordered_set<string> visited;
        visited.insert(start);
        queue<string> que;
        int curLevel = 1;
        int curLevelNum = 1;
        int nextLevelNum = 0;
        que.push(start);
        
        while (!que.empty())
        {
            string cur = que.front();
            que.pop();
            
            --curLevelNum;
            for (unordered_set<string>::iterator it = dict.begin(); it != dict.end(); ++it)
            {
                string temp = *it;
                if (canChange(temp, cur) && visited.find(temp) == visited.end())
                {
                    que.push(temp);
                    visited.insert(temp);
                    ++nextLevelNum;
                }
            }
            
            if (canChange(cur, end)) return curLevel + 1;
            
            if (curLevelNum == 0)
            {
                ++curLevel;
                curLevelNum = nextLevelNum;
                nextLevelNum = 0;                
            }
        }
        
        return 0;
    }
    
    bool canChange(string a, string b)
    {
        if (a.size() != b.size())
            return false;
        
        int count = 0;
        for (int i = 0; i < a.size(); ++i)
        {
            if (a[i] != b[i])            
            {
                if (count == 1) return false;
                ++count;
            }
        }
        
        return count == 0 ? false: true;
    }
};