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150. Evaluate Reverse Polish Notation

Description

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:

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

Example 2:

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

Example 3:

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].

Solutions

Solution 1

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import operator


class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        opt = {
            "+": operator.add,
            "-": operator.sub,
            "*": operator.mul,
            "/": operator.truediv,
        }
        s = []
        for token in tokens:
            if token in opt:
                s.append(int(opt[token](s.pop(-2), s.pop(-1))))
            else:
                s.append(int(token))
        return s[0]

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class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> stk = new ArrayDeque<>();
        for (String t : tokens) {
            if (t.length() > 1 || Character.isDigit(t.charAt(0))) {
                stk.push(Integer.parseInt(t));
            } else {
                int y = stk.pop();
                int x = stk.pop();
                switch (t) {
                case "+":
                    stk.push(x + y);
                    break;
                case "-":
                    stk.push(x - y);
                    break;
                case "*":
                    stk.push(x * y);
                    break;
                default:
                    stk.push(x / y);
                    break;
                }
            }
        }
        return stk.pop();
    }
}

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class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> stk;
        for (auto& t : tokens) {
            if (t.size() > 1 || isdigit(t[0])) {
                stk.push(stoi(t));
            } else {
                int y = stk.top();
                stk.pop();
                int x = stk.top();
                stk.pop();
                if (t[0] == '+')
                    stk.push(x + y);
                else if (t[0] == '-')
                    stk.push(x - y);
                else if (t[0] == '*')
                    stk.push(x * y);
                else
                    stk.push(x / y);
            }
        }
        return stk.top();
    }
};

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func evalRPN(tokens []string) int {
    // https://github.com/emirpasic/gods#arraystack
    stk := arraystack.New()
    for _, token := range tokens {
        if len(token) > 1 || token[0] >= '0' && token