TopCoder算法竞赛题5：SRM 149 DIV 2, 250-point

Problem Statement
In documents, it is frequently necessary to write monetary amounts in a standard format. We have decided to format amounts as follows:
the amount must start with '$'
the amount should have a leading '0' if and only if it is less then 1 dollar.
the amount must end with a decimal point and exactly 2 following digits.
the digits to the left of the decimal point must be separated into groups of three by commas (a group of one or two digits may appear on the left).
Create a class FormatAmt that contains a method amount that takes two int's, dollars and cents, as inputs and returns the properly formatted string.

Definition
Class: FormatAmt
Method: amount
Parameters: int, int
Returns: string
Method signature: string amount(int dollars, int cents)
(be sure your method is public)

Notes
One dollar is equal to 100 cents.

Constraints
dollars will be between 0 and 2,000,000,000 inclusive
cents will be between 0 and 99 inclusive

Examples
0)
123456
0
Returns: "$123,456.00"
Note that there is no space between the $ and the first digit.

1)
49734321
9
Returns: "$49,734,321.09"

2)
0
99
Returns: "$0.99"
Note the leading 0.

3)
249
30
Returns: "$249.30"

4)
1000
1
Returns: "$1,000.01"

Source code
#include<iostream>
#include<vector>
//#include<list>
//#include<map>
//#include<deque>
//#include<stack>
//#include<bitset>
//#include<algorithm>
//#include<functional>
//#include<numeric>
//#include<utility>
#include<sstream>
//#include<iomanip>
//#include<cstdio>
//#include<cmath>
//#include<cstdlib>
//#include<ctime>
using namespace std;

class FormatAmt
{
public:
string amount(int, int);
};

string FormatAmt::amount(int dollars, int cents)
{
string str = "";
int n = 0;
int a[100];
while(dollars > 0)
{
a[n++] = dollars%10;
dollars /= 10;
}

int count = 0;
for(int i=0; i<n; i++)
{
str = char(a[i]+48) + str;
count++;
if(count % 3 == 0)
str = "," + str;
}

if(str.length() == 0) str = "0";
if(str[0] == ',') str.erase(0, 1);
str = '$' + str + '.';
memset(a, 0, sizeof(a));
n = 0;
while(cents > 0)
{
a[n++] = cents % 10;
cents /= 10;
}
for(i=1; i>=0; i--)
{
str += char(a[i] + 48);
}

return str;
}

int main(void)
{
FormatAmt amt;
cout<<amt.amount(123456,0)<<endl;
cout<<amt.amount(49734321,9)<<endl;
cout<<amt.amount(0,99)<<endl;
cout<<amt.amount(100,1)<<endl;
return 0;
}