大数乘法模板

计算 a * b

时间复杂度为 O(n^2)

代码:

#include <iostream>
using namespace std;

const int MAXN = 1e4;
int a[MAXN], b[MAXN], sol[MAXN], lena, lenb;

void solve(void){
    int cnt = 0, indx = MAXN - 1;
    for(int i = lenb - 1; i >= 0; i--){
        int pos = indx--;
        for(int j = lena - 1; j >= 0; j--){
            int cc = b[i] * a[j] + cnt + sol[pos];
            sol[pos--] = cc % 10;
            cnt = cc / 10;
        }
        sol[pos] += cnt;
        cnt = 0;
    }
}

int main(void){
    string s1, s2;
    cin >> s1 >> s2;
    lena = s1.size();
    lenb = s2.size();
    for(int i = 0; i < lena; i++){
        a[i] = s1[i] - '0';
    }
    for(int i = 0; i < lenb; i++){
        b[i] = s2[i] - '0';
    }
    solve();
    int i = 0;
    while(sol[i] == 0) i++;
    while(i < MAXN) cout << sol[i++];
    cout << endl;
    return 0;
}