Can you say why build failed?

Question

#include<bits/stdc++.h>
using namespace std;
int main()
{
  int n, f, i, L, R, K, sum = 0, length, flag = 0;
  scanf("%d %d", &n, &f);
  char x[26];
  cout << "Enter a string: ";
  cin >> x;
  if(x[26] >= 'z' && x[26] <= 'a'){
     for(i=1; i<=f; i++){
        scanf("%d %d %d", &L, &R, &K);
        for(i=L; i<=R; i++){
           sum += i;
        }
     }
  }
  if(sum == K){
     if(x[i] != x[length-i-1]){
            flag = 1;
            break;
     }
  }

    if (flag) {
        cout << x << " no.." << endl;
    }
    else {
        cout << x << " yes.." << endl;
    }
    system("pause");
    return 0;
}

Answer 1

this is wrong "  if(x[26] >= 'z' && x[26] <= 'a')"you cant compare like 'a'>'b' or 'z'<='k' etc. this meanless and unuseful

you must be sure exactly  R>= L, "for(i=L; i<=R; i++)"