51nod 1027大数乘法

题目链接：51nod 1027大数乘法

直接模板了。

#include<cstdio>
#include<cstring>
using namespace std;
const int N = 1001;
const int DLEN = 4;
const int mod = 10000;
int alen, blen;
int ans_len;
char a1[N], b1[N];
int a[600], b[600];
int ans[600];

void BigInt(const char s[], int (&c)[600], int &len){
    memset(c, 0, sizeof(c));
    int L = strlen(s);
    len = L / DLEN;
    if(L%DLEN) len++;
    int cnt = 0;
    for(int i = L-1; i >= 0 ; i -= DLEN){
        int k = i - DLEN + 1;
        if(k < 0) k = 0;
        int t = 0;
        for(int j = k; j <= i; ++j)
            t = t * 10 + s[j] - '0';
        c[cnt++] = t;
    }
}
void multi(){
    int i, j;
    for(i = 0; i < alen; ++i){
        int up = 0;
        for(j = 0; j < blen; ++j){
            int t = a[i] * b[j] + ans[i+j] + up;
            up = t / mod;
            ans[i+j] = t % mod;
        }
        if(up != 0)
            ans[i+j] = up;
    }
    ans_len = alen + blen;
    while(ans[ans_len-1] == 0 && ans_len > 1)
        ans_len--;

    printf("%d", ans[ans_len-1]);
    for(i = ans_len-2; i >= 0; i--)
        printf("%04d", ans[i]);
    puts("");
}
int main(){
    scanf("%s%s", a1, b1);
    BigInt(a1, a, alen);
    BigInt(b1, b, blen);
    multi();
    return 0;
}