#include<stdio.h>
#include<string.h>
int main()
{
    char *key[]=
    {
        "",
        "",
        "ABC",
        "DEF",
        "GHI",
        "JKL",
        "MNO",
        "PQRS",
        "TUV",
        "WXYZ",
    };
    char output[100];
    void backtrack(char *digits,int index,int len)
    {
        if(index==len)
        {
            printf("%s",output);
            return;
        }
        int digit = digits[index] - '0';
    
    }