This is my code. I got WA 1 on G++ and G++11, but AC on Visual C++ 2010. Why?

#include <iostream>
const int M = 1000*1000+1;
char a[M], b[M];
int n;

using namespace std;

void read() {
    ios_base::sync_with_stdio(0);
    cin >> n;
    for(int i = n-1; i >= 0; --i) {
        cin >> a[i] >> b[i];
        a[i] -= 48;
        b[i] -= 48;
    }
}


void solve() {
    for(int i = 0; i < n; ++i) {
        a[i] += b[i];
        if(a[i] > 9) {
            a[i + 1]++;
            a[i] -= 10;
        }
    }
}

void print() {
    for(int i = n-1; i >= 0; --i) {
        cout << (int)a[i];
    }
}

int main() {
    read();
    solve();
    print();
}