problem0013.cpp

/*
* Output the first 4 digits of the sum of 100 50-digit numbers
*/

#include <iostream>
#include <string>

using namespace std;

string addRows(string, string);
string addZeroes(string, int);

int main(void) {
    string nums[100];
    string tmp;
    for (int i = 0; i < 100; i++) {
        cin >> tmp;
        tmp[tmp.length() - 1] = '\0';
        nums[i] = tmp;
    }

    string sum = addRows(nums[0], nums[1]);
    for (int j = 2; j < 100; j++) {
        sum = addRows(sum, nums[j]);
    }
    cout << sum << endl;
}

string addRows(string a, string b) {
    string sum;
    int carry = 0;
    int digit;
    char tmp;
    if (a.length() < b.length()) a = addZeroes(a, b.length() - a.length());
    if (b.length() < a.length()) b = addZeroes(b, a.length() - b.length());

    for (int i = a.length() - 1; i >= 0; i--) {
        digit = ((a[i] - '0') + (b[i] - '0') + carry) % 10;
        carry = ((a[i] - '0') + (b[i] - '0') + carry) / 10;
        tmp = digit + '0';
        sum = tmp + sum;
    }
    if (carry != 0) {
        tmp = carry + '0';
        sum = tmp + sum;
    }
    return sum;
}

string addZeroes(string num, int amount) {
    for (int i = 0; i < amount; i++) {
        num = '0' + num;
    }
    return num;
}