150.evaluate-reverse-polish-notation.java

/*
 * @lc app=leetcode id=150 lang=java
 *
 * [150] Evaluate Reverse Polish Notation
 *
 * https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
 *
 * algorithms
 * Medium (31.37%)
 * Total Accepted:    152.5K
 * Total Submissions: 484.1K
 * Testcase Example:  '["2","1","+","3","*"]'
 *
 * 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
 *
 *
 */
class Solution {
  public int evalRPN(String[] tokens) {
    Stack<Integer> stack = new Stack<>();
    for (String token : tokens) {
      if ("+".equals(token)) {
        stack.push(stack.pop() + stack.pop());
      } else if ("-".equals(token)) {
        stack.push(-stack.pop() + stack.pop());
      } else if ("*".equals(token)) {
        stack.push(stack.pop() * stack.pop());
      } else if ("/".equals(token)) {
        int b = stack.pop();
        int a = stack.pop();
        stack.push(a / b);
      } else {
        stack.push(Integer.valueOf(token));
      }
    }
    return stack.peek();
  }
}