0150. Evaluate Reverse Polish Notation

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

Description

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

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

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

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].

ac

// basic knowledge, stack
class Solution {
    public int evalRPN(String[] tokens) {
        // edge cases
        // if (tokens.length == 0) return -1;

        // stack store number
        Stack<Integer> stack = new Stack<Integer>();

        // walk list
        for (int i = 0; i < tokens.length; i++) {
            int res = 1;
            if (tokens[i].equals("+") || tokens[i].equals("-") 
                || tokens[i].equals("*") || tokens[i].equals("/")) {
                int operand = stack.pop();
                int number = stack.pop();
                switch (tokens[i]){
                    case "+": 
                        res = number + operand;
                        break;
                    case "-":
                        res = number - operand;
                        break;
                    case "*":
                        res = number * operand;
                        break;
                    case "/":
                        res = number / operand;
                        break;
                }
            } else {
                res = Integer.parseInt(tokens[i]);
            }
            stack.push(res);
        }
        // return 
        return stack.pop();
    }
}
// walk list, if number push to stack, else pop 2 numbers from stack -> calculate -> push back to stack
// stack1: 4 13 5

wrap operation in a helper function.

class Solution {
    public int evalRPN(String[] tokens) {
        // edge cases
        if (tokens == null || tokens.length == 0) return 0;

        Stack<Integer> stack = new Stack<>();
        for (String s : tokens) {
            if (s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) {
                int val2 = stack.pop();
                int val1 = stack.pop();
                stack.push(operate(val1, s, val2));
            } else {
                stack.push(Integer.parseInt(s));
            }
        }

        return stack.pop();
    }

    private int operate(int val1, String oper, int val2) {
        int res = 0;
        if (oper.equals("+")) res = val1 + val2;
        else if (oper.equals("-")) res = val1 - val2;
        else if (oper.equals("*")) res = val1 * val2;
        else if (oper.equals("/")) res = val1 / val2;
        return res;
    }
}