leetcode Evaluate Reverse Polish Notation

需要注意的是  访问栈顶元素使用stack.top(), stack.pop()只是删除栈顶元素并不返回栈顶元素

class Solution {
public:
   stack<int> dataStack;



void evalRPNHelper(string oPerator)
{
	int a = 0;
	int b = 0;
	b = dataStack.top();
	dataStack.pop();
	a = dataStack.top();
	dataStack.pop();
	switch (oPerator.at(0))
	{
	case '/':		
		dataStack.push(a/b);
		break;
	case '*':
		dataStack.push(a*b);
		break;
	case '+':
		dataStack.push(a+b);
		break;
	case '-':
		dataStack.push(a-b);
		break;
	default:
		break;
	}

}

void chartoint(string s)
{
	int flag = false;
	int data = 0;
	int sum = 0;
	int strlen = s.length();
	int t;
	if(s.at(0) == '-')
		flag = true;
	t = (flag)? 1:0; 

	for(int j = strlen-1; j >= t; --j)
	{
		data = s[j] - '0';
		data *= pow(10, strlen -1 -j);
		sum += data;
	}

	dataStack.push((flag)? -sum:sum);


}


int evalRPN(vector<string> &tokens)
{
	int strSize = tokens.size();
	for(int i = 0; i < strSize; ++i)
	{

		if(tokens.at(i)!="/"&&tokens.at(i)!= "-"&&tokens.at(i)!="+" && tokens.at(i)!="*")
			chartoint(tokens.at(i));
		else
			evalRPNHelper(tokens.at(i));

	}

	return dataStack.top();
}


};