【BFS】127. 单词接龙（待优化【建图】）

class Solution {
public:
    bool is_cross(string &word1, string &word2){
        bool flag = true;
        for(int i=0;i<word_length;++i)
            if(word1[i] != word2[i])
                if(flag)
                    flag = false;
                else
                    return false;
        return true;
    }

    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        word_length = beginWord.size();
        list_length = wordList.size();
        queue <int> s_q;
        int dp[list_length+1];
        bool is_go[list_length+1];
        for(int i=0;i<=list_length;++i){
            is_go[i] = false;
            dp[i] = 0;
        }
        s_q.push(0);
        is_go[0] = true;
        dp[0] = 1;
        while(!s_q.empty()){
            int cur_loc = s_q.front();
            s_q.pop();
            string cur_str;
            if(cur_loc == 0)
                cur_str = beginWord;
            else
                cur_str = wordList[cur_loc - 1];
            if(cur_str == endWord)
                return dp[cur_loc];
            for(int i=0;i<list_length;++i){
                if(is_cross(cur_str, wordList[i])){
                    if(dp[i+1] == 0)
                        dp[i+1] = dp[cur_loc] + 1;
                    else
                        dp[i+1] = min(dp[i+1],  dp[cur_loc]+1);
                    if(!is_go[i+1]){
                        is_go[i+1] = true;
                        s_q.push(i+1);
                    }
                }
            }
        }
        return 0;  
    }
private:
    int word_length;
    int list_length;
};

图（快了一丢丢）：

class Solution {
public:
    bool is_cross(string &word1, string &word2){
        bool flag = true;
        for(int i=0;i<word_length;++i)
            if(word1[i] != word2[i])
                if(flag)
                    flag = false;
                else
                    return false;
        return true;
    }

    void buildGraph(vector<string>& wordList){
        graph = vector<vector<int>>(list_length, vector<int>());
        for(int i=0;i<list_length;++i){
            for(int j=i+1;j<list_length;++j){
                if(is_cross(wordList[i],wordList[j])){
                    graph[i].push_back(j);
                    graph[j].push_back(i);
                    // cout << i << "," << j << "   ";
                }
            }
            // cout << endl;
        }
    }

    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        wordList.push_back(beginWord);
        list_length = wordList.size();
        word_length = beginWord.size();
        buildGraph(wordList);
        int dp[list_length];
        int beginIndex = list_length - 1;
        int endIndex = -1;
        for(int i=0;i<list_length;++i){
            dp[i] = -1;
            if(endWord == wordList[i])
                endIndex = i;
        }
        if(endIndex == -1)
            return 0;
        queue<int> q;
        dp[beginIndex] = 1;
        q.push(beginIndex);
        while(!q.empty()){
            int curIndex = q.front();
            q.pop();
            for(auto g : graph[curIndex]){
                if(dp[g] == -1){
                    dp[g] = dp[curIndex] + 1;
                    q.push(g); 
                } else {
                    dp[g] = min(dp[curIndex] + 1, dp[g]);
                }
            }
        }
        return dp[endIndex]==-1?0:dp[endIndex];
    }
private:
    int word_length;
    int list_length;
    vector<vector<int>> graph;
};