2021-05-22：假设所有字符都是小写字母， 大字符串是str，arr是去重的单词表， 每个单词都不是空字符串且可以使用任意

package main

import "fmt"

func main() {
    s := "aaab"
    arr := []string{"a", "aa", "aaa", "ab", "b"}
    ret := ways4(s, arr)
    fmt.Println(ret)
}

type Node struct {
    end   bool
    nexts []*Node
}

func NewNode() *Node {
    ans := &Node{}
    ans.nexts = make([]*Node, 26)
    return ans
}

func ways4(s string, arr []string) int {
    if len(s) == 0 || len(arr) == 0 {
        return 0
    }
    root := NewNode()
    for _, str := range arr {
        node := root
        index := 0
        for i := 0; i < len(str); i++ {
            index = int(str[i] - 'a')
            if node.nexts[index] == nil {
                node.nexts[index] = NewNode()
            }
            node = node.nexts[index]
        }
        node.end = true
    }

    N := len(s)
    dp := make([]int, N+1)
    dp[N] = 1
    for i := N - 1; i >= 0; i-- {
        cur := root
        for end := i; end < N; end++ {
            path := int(s[end] - 'a')
            if cur.nexts[path] == nil {
                break
            }
            cur = cur.nexts[path]
            if cur.end {
                dp[i] += dp[end+1]
            }
        }
    }
    return dp[0]
}