题目：

题解：

class Solution {
public:
    int minMutation(string start, string end, vector<string>& bank) {
        int m = start.size();
        int n = bank.size();
        vector<vector<int>> adj(n);
        int endIndex = -1;
        for (int i = 0; i < n; i++) {
            if (end == bank[i]) {
                endIndex = i;
            }
            for (int j = i + 1; j < n; j++) {
                int mutations = 0;
                for (int k = 0; k < m; k++) {
                    if (bank[i][k] != bank[j][k]) {
                        mutations++;
                    }
                    if (mutations > 1) {
                        break;
                    }
                }
                if (mutations == 1) {
                    adj[i].emplace_back(j);
                    adj[j].emplace_back(i);
                }
            }
        }
        if (endIndex == -1) {
            return -1;
        }

        queue<int> qu;
        vector<bool> visited(n, false);
        int step = 1;
        for (int i = 0; i < n; i++) {
            int mutations = 0;
            for (int k = 0; k < m; k++) {
                if (start[k] != bank[i][k]) {
                    mutations++;
                }
                if (mutations > 1) {
                    break;
                }
            }
            if (mutations == 1) {
                qu.emplace(i);
                visited[i] = true;
            }
        }        
        while (!qu.empty()) {
            int sz = qu.size();
            for (int i = 0; i < sz; i++) {
                int curr = qu.front();
                qu.pop();
                if (curr == endIndex) {
                    return step;
                }
                for (auto & next : adj[curr]) {
                    if (visited[next]) {
                        continue;
                    }
                    visited[next] = true;
                    qu.emplace(next);
                }
            }
            step++;
        }
        return -1;
    }
};