题目：

题解：

func ladderLength(beginWord string, endWord string, wordList []string) int {
    wordId := map[string]int{}
    graph := [][]int{}
    addWord := func(word string) int {
        id, has := wordId[word]
        if !has {
            id = len(wordId)
            wordId[word] = id
            graph = append(graph, []int{})
        }
        return id
    }
    addEdge := func(word string) int {
        id1 := addWord(word)
        s := []byte(word)
        for i, b := range s {
            s[i] = '*'
            id2 := addWord(string(s))
            graph[id1] = append(graph[id1], id2)
            graph[id2] = append(graph[id2], id1)
            s[i] = b
        }
        return id1
    }

    for _, word := range wordList {
        addEdge(word)
    }
    beginId := addEdge(beginWord)
    endId, has := wordId[endWord]
    if !has {
        return 0
    }

    const inf int = math.MaxInt64
    distBegin := make([]int, len(wordId))
    for i := range distBegin {
        distBegin[i] = inf
    }
    distBegin[beginId] = 0
    queueBegin := []int{beginId}

    distEnd := make([]int, len(wordId))
    for i := range distEnd {
        distEnd[i] = inf
    }
    distEnd[endId] = 0
    queueEnd := []int{endId}

    for len(queueBegin) > 0 && len(queueEnd) > 0 {
        q := queueBegin
        queueBegin = nil
        for _, v := range q {
            if distEnd[v] < inf {
                return (distBegin[v]+distEnd[v])/2 + 1
            }
            for _, w := range graph[v] {
                if distBegin[w] == inf {
                    distBegin[w] = distBegin[v] + 1
                    queueBegin = append(queueBegin, w)
                }
            }
        }

        q = queueEnd
        queueEnd = nil
        for _, v := range q {
            if distBegin[v] < inf {
                return (distBegin[v]+distEnd[v])/2 + 1
            }
            for _, w := range graph[v] {
                if distEnd[w] == inf {
                    distEnd[w] = distEnd[v] + 1
                    queueEnd = append(queueEnd, w)
                }
            }
        }
    }
    return 0
}