string_addReverseNumber.c

#include <stdio.h>
#include <string.h>
#define N 210
int main()
{
    char s1[N], s2[N], s3[N];
    int ncase, i, length1, length2, maxlength;
    freopen("string_addReverseNumber.in", "r", stdin);
    // freopen("my.out","w",stdout);
    scanf("%d", &ncase);
    while (ncase--) {
        scanf("%s%s", s1, s2);
        length1 = strlen(s1);
        length2 = strlen(s2);

        for (i = 0; i < N; i++) // initialize s3 with 0
            s3[i] = 0;

        if (length1 > length2) {
            for (i = length2; i < length1; i++) {
                s2[i] = '0';
            }
            s2[length1] = '\0';
            maxlength = length1;
        } else {
            for (i = length1; i < length2; i++) {
                s1[i] = '0';
            }
            s1[length2] = '\0';
            maxlength = length2;
        }

        for (i = 0; i < maxlength; i++) {
            s3[i] = ((s1[i] - '0') + (s2[i] - '0'));
        }

        for (i = 0; i < maxlength; i++) {
            if (s3[i] > 9) {
                s3[i + 1] += 1;
                s3[i] %= 10;
            }
            s3[i] += '0'; // do int to char
        }

        if (s3[maxlength] != 0) { // don't forget the last carry, e.g. 9999 + 9999
            // -> you will have 5 digits!!
            s3[maxlength] += '0';
            maxlength++;
            s3[maxlength] = '\0';
        }

        /*
        s3[maxlength] = 0;
        if (s3[maxlength - 1] > 9) {
        s3[maxlength] += 1;
        s3[maxlength - 1] %= 10;
        s3[maxlength + 1] = '\0';
        for (i = 0; i <= maxlength; i++) {
        s3[i] += '0';
        }
        } else {
        s3[maxlength] = '\0';
        for (i = 0; i < maxlength; i++)
        s3[i] += '0';
        }
        */

        // find the starting point!
        int start = 0;
        for (i = 0; i < N; i++) {
            if (s3[i] == '0' || s3[i] == '\0')
                continue;
            start = i;
            break;
        }

        printf("%s\n", s3 + start);
    }
    return 0;
}