[C++知识库]大数相加，相乘，阶乘（c++实现）

大数相加

#include<bits/stdc++.h>
using namespace std;

int main() {
	string a, b,ans;
	cin >> a >> b;
	int len1 = a.length();
	int len2 = b.length();
	if ((len1-len2)>0) {
		for (int i = 0; i < (len1-len2); i++) {
			b = '0' + b;
		}
	}
	else {
		for (int i = 0; i <(len2-len1); i++) {
			a = '0' + a;
		}
	}
	int temp;
	int len = a.length();
	int cf = 0;
	for (int i =len-1 ; i >=0 ; i--) {
		temp = a[i] - '0' + b[i] - '0' + cf;
		cf = temp / 10;
		temp %= 10;
		ans = (char)(temp + '0') + ans;
	}
	if (cf != 0) ans = (char)(cf + '0') + ans;
	cout << ans<<endl;
}

大数相乘

int main() {
	string aa, bb;
	int a[10001], b[10001], ans[10001] = { 0 };
	cin >> aa >> bb;
	int len1 = aa.length();
	int len2 = bb.length();
	for (int i = 0; i <= len1 - 1; i++) a[i] = aa[len1 - 1 - i] - '0';
	for (int i = 0; i <= len2 - 1; i++) b[i] = bb[len2 - 1 - i] - '0';
	for (int i = 0; i < len1; i++) {
		for (int j = 0; j < len2; j++) {
			ans[i + j] += a[i] * b[j];
		}
	}
	for (int i = 0; i < len1 + len2 - 1; i++) {
		if (ans[i] > 9) {
			ans[i + 1] += ans[i] / 10;
			ans[i] %= 10;
		}
	}
	int len = len1 + len2-1;
	while (ans[len]==0 && len > 0) len--;
	for (int i = len; i >= 0; i--) cout << ans[i];
}

大数乘小数

void solve(int* k, int b) {
	for (int i = 0,c=0; i < L; i++) {
		c = c / 10 + k[i]*b;
		k[i] = c % 10;
	}
}
//K[0]=1；
//为一个大数，b为要乘的小数

n大于20的阶乘

string mut(string num, int n) {
	string res;
	int c = 0;
	for (int i = num.size() - 1; i >= 0; i--) {
		int temp = (num[i] - '0') * n + c;
		c = temp / 10;
		temp = temp % 10;
		res.insert(res.begin(), temp + '0');
	}
	while (c != 0) {
		char temp = c % 10 + '0'; res = temp + res;
		c /= 10;
	}
	return res;
}
string solve(int n) {
	if (n == 0 || n == 1) {
		return "1";
	}
	return mut(solve(n - 1), n);
}