bigsum.c

#include<stdio.h>
#include<string.h>

int main()
{
    char num1[500];
    char num2[500];
    scanf("%s",num1);
    scanf("%s",num2);

    char sum[501];
    int len1 = strlen(num1);
    int len2 = strlen(num2);

    if(len1>=len2)
    {
        int i = len1-1;
        int j = len2-1;
        int carryIn = 0;
        for(;i>=0;i--)
        {
            int temp;
            if(j<0) temp = (num1[i] - '0') + carryIn;
            else temp = ((num1[i] - '0') + (num2[j] - '0')) + carryIn;
            sum[i+1] = temp%10 + '0';
            if(temp/10 == 1) carryIn = 1;
            j--;
        }
        if(carryIn) sum[0] = '1';
        else sum[0] = ' ';
        sum[len1+1]='\0';

    }
    else
    {
        int i = len1-1;
        int j = len2-1;
        int carryIn = 0;
        for(;j>=0;j--)
        {
            int temp;
            if(i<0) temp = (num2[j] - '0') + carryIn;
            else temp = ((num1[i] - '0') + (num2[j] - '0')) + carryIn;
            sum[j+1] = temp%10 + '0';
            if(temp/10 == 1) carryIn = 1;
            i--;
        }
        if(carryIn) sum[0] = '1';
        else sum[0] = ' ';
        sum[len2+1]='\0';
    }

    printf("number1 : %s \nnumber2 : %s\nsummation : %s\n",num1,num2,sum);
}