题目：

题解：

class Solution {
    public int minMutation(String start, String end, String[] bank) {
        int m = start.length();
        int n = bank.length;
        List<Integer>[] adj = new List[n];
        for (int i = 0; i < n; i++) {
            adj[i] = new ArrayList<Integer>();
        }
        int endIndex = -1;
        for (int i = 0; i < n; i++) {
            if (end.equals(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].charAt(k) != bank[j].charAt(k)) {
                        mutations++;
                    }
                    if (mutations > 1) {
                        break;
                    }
                }
                if (mutations == 1) {
                    adj[i].add(j);
                    adj[j].add(i);
                }
            }
        }
        if (endIndex == -1) {
            return -1;
        }

        Queue<Integer> queue = new ArrayDeque<Integer>();
        boolean[] visited = new boolean[n];
        int step = 1;
        for (int i = 0; i < n; i++) {
            int mutations = 0;
            for (int k = 0; k < m; k++) {
                if (start.charAt(k) != bank[i].charAt(k)) {
                    mutations++;
                }
                if (mutations > 1) {
                    break;
                }
            }
            if (mutations == 1) {
                queue.offer(i);
                visited[i] = true;
            }
        }        
        while (!queue.isEmpty()) {
            int sz = queue.size();
            for (int i = 0; i < sz; i++) {
                int curr = queue.poll();
                if (curr == endIndex) {
                    return step;
                }
                for (int next : adj[curr]) {
                    if (visited[next]) {
                        continue;
                    }
                    visited[next] = true;
                    queue.offer(next);
                }
            }
            step++;
        }
        return -1;
    }
}