150. Evaluate Reverse Polish Notation

Description

Evaluate the value of an arithmetic expression in Reverse Polish Notation(or 逆波兰表达式).

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

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won’t be any divide by zero operation.

Example 1:

Input: [“2”, “1”, “+”, “3”, “*”]

Output: 9

Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: [“4”, “13”, “5”, “/”, “+”]

Output: 6

Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: [“10”, “6”, “9”, “3”, “+”, “-11”, “*”, “/”, “*”, “17”, “+”, “5”, “+”]

Output: 22

Explanation:

((10 * (6 / ((9 + 3) * -11))) + 17) + 5

= ((10 * (6 / (12 * -11))) + 17) + 5

= ((10 * (6 / -132)) + 17) + 5

= ((10 * 0) + 17) + 5

= (0 + 17) + 5

= 17 + 5

= 22

Idea

Stack.

Solution

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