solution.cpp

#include <stdio.h>
void bigmul(int x, int y, int r[])
{
    int base = 10000;
    int x2 = x / base;
    int x1 = x % base;
    int y2 = y / base;
    int y1 = y % base;

    int n1 = x1 * y1;
    int n2 = x1 * y2;
    int n3 = x2 * y1;
    int n4 = x2 * y2;

    r[3] = n1 % base;
    r[2] = n1 / base + n2 % base + n3 % base;
    r[1] = n2 / base + n3 / base + n4 % base; // 填空
    r[0] = n4 / base;

    r[1] += r[2] / base; // 填空
    r[2] = r[2] % base;
    r[0] += r[1] / base;
    r[1] = r[1] % base;
}

int main(int argc, char *argv[])
{
    int x[] = {0, 0, 0, 0};

    bigmul(87654321, 12345678, x);

    printf("%d%d%d%d\n", x[0], x[1], x[2], x[3]);

    return 0;
}