[LeetCode] Evaluate Reverse Polish Notation

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.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

Solution: Use a stack.

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public class Solution { private boolean isNumber(String s) { if (s.charAt(0) == '-' && s.length() > 1) { return true; } if (s.charAt(0) >= '0' && s.charAt(0) <= '9') { return true; } return false; } private int evalOp(int first, int second, String s) { if (s.equals("+")) { return first + second; } if (s.equals("-")) { return first - second; } if (s.equals("*")) { return first * second; } if (s.equals("/")) { return first / second; } return 0; } public int evalRPN(String[] tokens) { Stack<Integer> stack = new Stack<Integer>(); for (String s : tokens) { if (isNumber(s)) { stack.push(Integer.parseInt(s)); } else { int second = stack.pop(); int first = stack.pop(); int result = evalOp(first, second, s); stack.push(result); } } return stack.pop(); } }