A + B Problem II HDU - 1002（简单string）

#include <iostream> 
#include <string>
#include <algorithm>

using namespace std;

string sum(string a, string b)
{
	if(a.length() < b.length())
	{
		string tmp = a;
		a = b;
		b = tmp;
	}
	string tmp = "";
	for(int i = 0; i < a.length()-b.length(); i++)
		tmp += "0";
	b = tmp + b;    //以上操作都是对齐操作。
	
	int x;
	bool flag = false;
	string ans = "";
	for(int i = a.length()-1; i >= 0; i--)
	{
		x = a[i] - '0' + b[i] - '0' + flag;
		if(x >= 10)
		{
			flag = true;
			ans += '0' + x - 10;
		}
		else
		{
			flag = false;
			ans += x + '0';
		}	
	} 
	if(flag)   //注意这里，比如99 + 11, 和是3位，
		ans += "1";
	reverse(ans.begin(), ans.end());
	return ans;
}

int main()
{
	string str1, str2, ans;
	int n;
	cin >> n;
	for(int i = 1; i <= n; i++)
	{
		cin >> str1 >> str2;
		cout << "Case " << i << ":" << endl;
		cout << str1 << " + " << str2 << " = " << sum(str1, str2) << endl;
		if(i != n)
			cout << endl;
	}
	return 0;
}