150. Evaluate Reverse Polish Notation

Posted on Edited on In

150. Evaluate Reverse Polish Notation

Description

Difficulty: Medium

Related Topics: Array, Math, Stack

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

  • The valid operators are '+', '-', '*', and '/'.
  • Each operand may be an integer or another expression.
  • The division between two integers always truncates toward zero.
  • There will not be any division by zero.
  • The input represents a valid arithmetic expression in a reverse polish notation.
  • The answer and all the intermediate calculations can be represented in a 32-bit integer.

Example 1:

1
2
3
Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

1
2
3
Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

1
2
3
4
5
6
7
8
9
Input: tokens = ["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

Constraints:

  • 1 <= tokens.length <= 104
  • tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Hints/Notes

  • 2023/11/30
  • stack
  • No solution from 0x3F

Solution

Language: C++

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> s;
        for (string token : tokens) {
            if (token == "+") {
                int a = s.top(); s.pop();
                int b = s.top(); s.pop();
                s.push(a + b);
            } else if (token == "-") {
                int a = s.top(); s.pop();
                int b = s.top(); s.pop();
                s.push(b - a);
            } else if (token == "*" ) {
                int a = s.top(); s.pop();
                int b = s.top(); s.pop();
                s.push(a * b);
            } else if (token == "/") {
                int a = s.top(); s.pop();
                int b = s.top(); s.pop();
                s.push(b / a);
            } else {
                s.push(stoi(token));
            }
        }
        return s.top();
    }
};