LeetCode - Evaluate Reverse Polish Notation

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

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

用 stack 完美契合

class Solution {
    public int evalRPN(String[] tokens) {
        if(tokens == null || tokens.length == 0){
            return 0;
        }
        
        Stack<Integer> stack = new Stack<>();
        
        for(String s : tokens){
            if(s.equals("+")){
                int first = stack.pop();
                int second = stack.pop();
                int res =  second + first;
                stack.add(res);
                
            }
            else if(s.equals("-")){
                int first = stack.pop();
                int second = stack.pop();
                int res = second - first;
                stack.add(res);
            }
            else if(s.equals("*")){
                int first = stack.pop();
                int second = stack.pop();
                int res = second * first;
                stack.add(res);
            }
            else if(s.equals("/")){
                int first = stack.pop();
                int second = stack.pop();
                int res = second / first;
                stack.add(res);
            }
            else{
                stack.add(Integer.parseInt(s));
            }
        }
        
        return stack.pop();
    }
}