[leetcode] 150. Evaluate Reverse Polish Notation 解题报告

题目链接：https://leetcode.com/problems/evaluate-reverse-polish-notation/

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

思路：这是将运算符后置的方式，当然可以想到用栈来做，如果是数字就入栈，如果是运算符就取出栈顶的两个元素，运算之后将结果再压栈，最后栈顶元素就是最终的结果。

代码如下：

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