HDU-1161-Eddy's mistakes

Eddy’s mistakes

Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 10182 Accepted Submission(s): 5736

Problem Description
Eddy usually writes articles ,but he likes mixing the English letter uses, for example “computer science” is written frequently “coMpUtEr scIeNce” by him, this mistakes lets Eddy’s English teacher be extremely discontentment.Now please you to write a procedure to be able in the Bob article English letter to turn completely the small letter.

Input
The input contains several test cases.each line consists a test case，Expressed Eddy writes in an article , by letter, blank space,numeral as well as each kind of punctuation
composition, the writing length does not surpass 1000 characters.

Output
For each test case, you should output an only line, after namely the result of transforms the lowercase letter.

Sample Input
weLcOmE tO HDOj Acm 2005!

Sample Output
welcome to hdoj acm 2005!

题意：输入一行，如果是小写，原样输出，如果是大写，转换成小写再输出

我知道很水，我只是为了练习string
string用法http://blog.csdn.net/qq_32680617/article/details/51122395
代码

#include<stdio.h>
#include<string.h>
#include<string>
#include<algorithm>
#include<iostream>
#include<math.h>
#include<stack>
#include<cctype>
using namespace std;
int main()
{
    string s1;
    while(getline(cin,s1))
    {
        for(string::size_type i=0; i<s1.size(); i++)
        {
            char c=tolower(s1[i]);//直接输出会报错
            cout<<c;
        }
        cout<<endl;
    }
    return 0;
}