2021-02-17：规定1和A对应、2和B对应、3和C对应...26和Z对应

package main

import "fmt"

func main() {
    str := "7210231231232031203123"
    fmt.Println("1.递归：", number1(str))
    fmt.Println("2.动态规划：", number2(str))
}
func number1(str string) int {
    if len(str) == 0 {
        return 0
    }
    return process1(str, 0)
}
func process1(str string, index int) int {
    strLen := len(str)
    if strLen == index { //1
        return 1
    }
    if str[index] == '0' {
        return 0
    }
    ret := process1(str, index+1)
    if index+1 < strLen && (str[index] == '1' || (str[index] == '2' && str[index+1] <= '6')) {
        ret += process1(str, index+2)
    }
    return ret
}

func number2(str string) int {
    strLen := len(str)
    if strLen == 0 {
        return 0
    }
    dp := make([]int, strLen+1)
    dp[strLen] = 1 //1
    for i := strLen - 1; i >= 0; i-- {
        if str[i] == '0' {
            continue
        }
        dp[i] = dp[i+1]
        if i+1 < strLen && (str[i] == '1' || (str[i] == '2' && str[i+1] <= '6')) {
            dp[i] += dp[i+2]
        }
    }
    return dp[0]
}