题目：

题解：

func diffOne(s, t string) (diff bool) {
    for i := range s {
        if s[i] != t[i] {
            if diff {
                return false
            }
            diff = true
        }
    }
    return
}

func minMutation(start, end string, bank []string) int {
    if start == end {
        return 0
    }

    m := len(bank)
    adj := make([][]int, m)
    endIndex := -1
    for i, s := range bank {
        if s == end {
            endIndex = i
        }
        for j := i + 1; j < m; j++ {
            if diffOne(s, bank[j]) {
                adj[i] = append(adj[i], j)
                adj[j] = append(adj[j], i)
            }
        }
    }
    if endIndex == -1 {
        return -1
    }

    var q []int
    vis := make([]bool, m)
    for i, s := range bank {
        if diffOne(start, s) {
            q = append(q, i)
            vis[i] = true
        }
    }
    for step := 1; q != nil; step++ {
        tmp := q
        q = nil
        for _, cur := range tmp {
            if cur == endIndex {
                return step
            }
            for _, nxt := range adj[cur] {
                if !vis[nxt] {
                    vis[nxt] = true
                    q = append(q, nxt)
                }
            }
        }
    }
    return -1
}