如何添加运算符

package main

import (
    "fmt"
    "strconv"
)

func main() {
    var n,m int
    fmt.Scan(&n,&m)
    ans:=0
    var dfs func(string,int)
    dfs=func(s string,idx int){
        if idx==n+1{
            if cal(s)==m{
                ans++
            }
            return
        }
        dfs(s+"+"+strconv.Itoa(idx),idx+1)
        dfs(s+"-"+strconv.Itoa(idx),idx+1)
        dfs(s+strconv.Itoa(idx),idx+1)
    }
    dfs("1",2)
    fmt.Print(ans)
}

func cal(s string)int{
    arr1,arr2:=[]int{},[]byte{}
    pre:=""
    for _,ch:=range []byte(s){
        if ch=='+'||ch=='-'{
            x,_:=strconv.Atoi(pre)
            arr1=append(arr1,x)
            arr2=append(arr2,ch)
            pre=""
        }else{
            pre+=string(ch)
        }
    }
    x,_:=strconv.Atoi(pre)
    arr1=append(arr1,x)
    ans:=arr1[0]
    for i:=1;i<len(arr1);i++{
        if arr2[i-1]=='+'{
            ans+=arr1[i]
        }else{
            ans-=arr1[i]
        }
    }
    return ans
}