Could anyone tell where my logic / testcase in this problem is wrong?

Problem link: Contest Page | CodeChef
Approach: Used binary search on the index at which the number of operations are counted.
Could anyone please tell me which test case was I missing?

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <set>
using namespace std;
bool check(string str, int val, int k){
    int cnt =0, i;
    for(i=val;i>=0;i--){
        if(str[i] == '0'){
            continue;
        }
        int num = str[i] - '0';
        num = num + cnt%10;
        num = num%10;
        str[i] = '0' + num;
        cnt = cnt + 10  - (str[i] - '0');
        
    }
    if(cnt > k){
        return false;
    }
    return true;
}
int main(){
    int t;
    cin>>t;
    
 
    while(t--){
        int n, k, i ,j;
        cin >>n >> k;
        string str;
        cin>>str;
        int left = 0, right = n-1, mid = left + (right - left)/2;
        if(n==1){
            int num = str[0]-'0';
            if(10 - num <= k ){
                cout<<"1"<<endl;
            }
            else{
                cout<<"0"<<endl;
            }
        }
        else if(n ==2){
            int num1 = 10 - str[0]+ '0';
            int num2 = 10 - str[1] + '0';
            int count = 0;
            while(num1 + 10 <= k){
                num1 = num1 +10;
            }
            while(num2 + 10 <= num1){
                num2 = num2 +10;
            }
            if(num1 > k){
                cout<<"0"<<endl;
            }
            else if(num1>=num2){
                cout<<"2"<<endl;
            }
            else{
                cout<<"1"<<endl;
            }
        }else{
        while(right - left > 1){
            mid = left + (right - left)/2;
            if(check(str,mid,k)){
                left = mid;
            }
            else{
                right = mid -1;
            }
        }
        if(!check(str, 0, k)){ // checking if no index can be made zero
            int f, c =0 ;// the initial zeros already present in given string
            for(f=0;f<n;f++){
              if(str[f] != '0'){
                  break;
              }else{
                  c++;
              }
            }
            cout<<c<<endl;
        }
        else if(check(str, right, k)){
            cout<<right + 1<<endl;
        }
        else{
            cout<<left + 1<<endl;
        }
        
        }
    }
}

Please help!