LeetCode150 Evaluate Reverse Polish Notation

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

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

分析：

根据逆波兰表达式定义，建立栈存数字

遍历字符串，遇到数字，入栈，遇到运算符，取出栈顶两元素执行相应计算后将结果入栈

代码：

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> s;
        for (int i = 0;i < tokens.size();i++) {
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i]== "*" || tokens[i] == "/") {
                int num1 = s.top();
                s.pop();
                int num2 = s.top();
                s.pop();
                if (tokens[i] == "+") {
                    int num = num1 + num2;
                    s.push(num);
                }
                if (tokens[i] == "-") {
                    int num = num2 - num1;
                    s.push(num);
                }
                if (tokens[i] == "*") {
                    int num = num2 * num1;
                    s.push(num);
                }
                if (tokens[i] == "/") {
                    int num = num2 / num1;
                    s.push(num);
                }
            }
            else {
                s.push(atoi(tokens[i].c_str()));
            }
        }
        int ans = s.top();
        return ans;
    }
};