题解 | #Powerful Calculator#

#include<iostream>
#include<string>
using namespace std;
int main(){
	string str1, str2;
	while (cin >> str1 >> str2){
		int s1 = str1.size();
		int s2 = str2.size();
		if (s1 > s2){
			for (int i = 0; i < s1 - s2; i++){
				str2.insert(0,"0");
			}
		}
		if (s2 > s1){
			for (int i = 0; i < s2 - s1; i++){
				str1.insert(0, "0");
			}
		}
		string add = str1;
		string sub = str1;
		int k=0, m;
		for (int i = add.size() - 1; i >= 0; i--){  //加法
			m = add[i] - '0' + str2[i] - '0' + k;
			k = m / 10;
			add[i] = m % 10 + '0';
		}
		if (k>0){
			add.insert(0,"1");
		}
		cout << add << endl;
		if (s1 > s2 || s1 == s2&&str1 > str2){
			for (int i = sub.size() - 1; i >= 0; i--){    //减法
				if (sub[i] >= str2[i]){
					sub[i] = sub[i] - str2[i] + '0';
				}
				else{
					sub[i] = 10 + sub[i] - str2[i] + '0';
					sub[i - 1]--;
				}
			}
			while (sub[0] == '0'&&sub.size()>1){
				sub.erase(0, 1);
			}
		}
		else{
			sub = str2;
			for (int i = sub.size() - 1; i >= 0; i--){    //减法
				if (sub[i] >= str1[i]){
					sub[i] = sub[i] - str1[i] + '0';
				}
				else{
					sub[i] = 10 + sub[i] - str1[i] + '0';
					sub[i - 1]--;
				}
			}
			while (sub[0] == '0'&&sub.size()>1){
				sub.erase(0, 1);
			}
			sub.insert(0, "-");
		}
		cout<< sub << endl;
		string ans;     //最终结果
		string mut;     //中间结果
		for (int i = str1.size() - 1; i >= 0; i--){    //乘法
			int x = 0, y = 0;
			for (int j = str2.size()-1; j >= 0; j--){
				x = (str1[i] - '0')*(str2[j] - '0') + y;
				y = x / 10;
				mut.insert(0, 1, x % 10 + '0');
			}
			if (y > 0){
				mut.insert(0, 1,y+'0');
			}
			mut.insert(mut.size(), str1.size() - 1 - i, '0');
			if (i == str1.size()-1){
				ans = mut;
			}
			else{
				int z1 = mut.size(), z2 = ans.size();
				if (z1 > z2){
					for (int i = 0; i < z1 - z2; i++){
						ans.insert(0, "0");
					}
				}
				if (z2 > z1){
					for (int i = 0; i < z2 - z1; i++){
						mut.insert(0, "0");
					}
				}
				int p = 0, q = 0;
				for (int i = ans.size() - 1; i >= 0; i--){  //中间结果加最终结果
					p = ans[i] - '0' + mut[i] - '0' + q;
					q = p / 10;
					ans[i] = p % 10 + '0';
				}
				if (q>0){
					ans.insert(0, "1");
				}
			}
			mut.clear();
		}
		while (ans[0] == '0'&&ans.size()>1){
			ans.erase(0, 1);
		}
		cout << ans << endl;
	}
}