解題思路：碰到數字入棧，碰到運算子，推出棧頂兩個元素進行相應運算，將結果放回棧中。

class Solution {
	public:
		int evalRPN(vector<string>& tokens) {
			stack<int> nums;
			for (int i = 0; i < tokens.size(); i++) {
				if (tokens[i] == "+" || tokens[i] == "-"
						|| tokens[i] == "/" || tokens[i] == "*") {
					int a = nums.top(); nums.pop();
					int b = nums.top(); nums.pop();
					if (tokens[i] == "+") nums.push(b + a);
					else if(tokens[i] == "-") nums.push(b - a);
					else if (tokens[i] == "*") nums.push(b * a);
					else if (tokens[i] == "/") nums.push(b / a);
				} else {
					nums.push(stoi(tokens[i]));
				}
			}
			return nums.top();
		}
};