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
The concept is:
When meeting the number, push into the stack.
When meeting the operator, pop the top 2 number and compute the value, then push the result back into the stack.
Until the end of the expression.
Output the top (last) value in the stack.
public class Solution { public int evalRPN(String[] tokens) { Stack<Integer> stack = new Stack<Integer>(); if(tokens == null || tokens.length == 0) return 0; for(int i = 0; i < tokens.length; i++){ String str = tokens[i]; if(str.equals("+")){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num1 + num2); } else if(str.equals("-")){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num2 - num1); } else if(str.equals("*")){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num1 * num2); } else if(str.equals("/")){ int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num2 / num1); } else stack.push(Integer.valueOf(str)); } return stack.pop(); } }
Evaluate Reverse Polish Notation
原文：http://www.cnblogs.com/RazerLu/p/3557483.html