高精度整數的乘法

//高精度整數的乘法
#include<iostream>
using namespace std;

int Insert(int* a, int na, int x) {
	for (int i = na; i > 0; i--)
		a[i] = a[i - 1];
	a[0] = x;
	return na + 1;
}

int Mul(int* a, int na, int x) {
	int i;
	int jw = 0;	//進位

	for (i = 0; i < na; i++) {
		int temp = a[i] * x + jw;
		a[i] = temp % 10;
		jw = temp / 10;
	}

	//進位不為0的時候處理a[na]及以後的位
	while (jw != 0) {
		a[i] = jw % 10;
		i++;
		jw = jw / 10;
	}
	return i;
}

int Mult(int* a, int na, int* b, int nb, int* c) {
	int d[100][100];
	int i, j, k;
	int m = 0;

	//初始化為0
	for (i = 0; i < 100; i++)
		for (j = 0; j < 100; j++)
			d[i][j] = 0;

	//用a[1], a[2], ... a[na - 1]來乘以b陣列中每一個數
	//a[i]乘以後的結果存放在 d[i]中
	for (i = 0; i < na; i++) {

		//先賦值
		for (j = 0; j < nb; j++)
			d[i][j] = b[j];

		m = nb;

		//根據i處在什麼位置插入0
		//i為個位,插入0個0
		//i為十位,插入一個0
		for (k = 0; k < i; k++)
			m = Insert(d[i], m, 0);

		//把d[i]的所有元素乘以a[i]
		for (k = 0; k < m; k++)
			d[i][k] *= a[i];
	}

	//相加
	for (i = 0; i < m; i++)
		for (j = 0; j < na; j++)
			c[i] += d[j][i];


	//進位
	for (i = 0; i < m; i++) {
		c[i + 1] += c[i] / 10;
		c[i] = c[i] % 10;
	}

	//進位
	while (c[i] > 10) {
		c[i + 1] = c[i] / 10;
		c[i] = c[i] % 10;
		i++;
	}
	return i;
}

int main() {
	int a[20] = { 0 }, na;
	int b[20] = { 0 }, nb;
	int c[40] = { 0 }, nc;
	int x1, x2;
	int n1, n2;
	int i;
	a[0] = b[0] = 1;
	na = nb = 1;

	cin >> x1 >> n1 >> x2 >> n2;
	for (i = 0; i < n1; i++)
		na = Mul(a, na, x1);

	for (i = 0; i < n2; i++)
		nb = Mul(b, nb, x2);

	nc = Mult(a, na, b, nb, c);

	for (i = nc - 1; i >= 0; i--)
		cout << c[i] << ' ';
}

實現過程:

最後進位即可得到結果

缺點: a, b, c和d陣列的所有元素都要為0
不然出現隨機數