Assuming I understand what the code is supposed to do, I would start by simplify the code:
<pre>`\#include \<bits/stdc++.h\>
using namespace std;
int main(){
int N;
cin \>\> N;
int sum = 0;
vector\<int\> a(N);
int A, B;
cin \>\> A \>\> B;
for (int i = 0; i \< N; i++) {
a\[i\] = i + 1;
int num = a\[i\];
int digitsum = 0;
if((a[i] >= A) && (a[i] <= B)) {
sum = sum + a[i];
} else if (a[i] > 0) {
while (a[i] > 0) {
digitsum += a[i] % 10;
a[i] = a[i] / 10;
}
if (digitsum >= A && digitsum <= B) {
sum = sum + num;
}
}
}
cout \<\< sum \<\< endl;
}
`
12 is not the sum of its digits.