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
思路:用一个栈便可以解决。
package com.leetcode;
import java.util.Scanner;
import java.util.Stack;
/**
* @author YanQiKing
* @date 2019/8/20 8:17
* leetcode 150 逆波兰表达式求值
*/
public class ReversePolish {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String s = sc.nextLine().trim();
String[] tokens = s.split(" ");
System.out.println(evalRPN(tokens));
}
public static int evalRPN(String[] tokens) {
Stack<Integer> stack = new Stack<>();
for (int i = 0; i < tokens.length;i++) {
if (tokens[i].equals("+") || tokens[i].equals("-") || tokens[i].equals("*") || tokens[i].equals("/")) {
int a = stack.pop();
int b = stack.pop();
if (tokens[i].equals("+") ) {
stack.add(a+b);
}else if (tokens[i].equals("-") ){
stack.add(b - a);
}else if (tokens[i].equals("*")) {
stack.add(b * a);
}else {
stack.add(b /a);
}
}else {
stack.add(Integer.parseInt(tokens[i]));
}
}
return stack.peek();
}
}