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[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.
Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9 ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/
思路：stack的经典题目，遇到数字压栈，遇到符号出栈两个计算再入栈。
import java.util.Stack;public class Solution { public int evalRPN(String[] tokens) { Stack<Integer> stack = new Stack<Integer>(); int a, b; for (int i = 0; i < tokens.length; i++) { if (tokens[i].equals("+")) { a = stack.pop(); b = stack.pop(); stack.push(b + a); } else if (tokens[i].equals("-")) { a = stack.pop(); b = stack.pop(); stack.push(b - a); } else if (tokens[i].equals("*")) { a = stack.pop(); b = stack.pop(); stack.push(b * a); } else if (tokens[i].equals("/")) { a = stack.pop(); b = stack.pop(); stack.push(b / a); } else { stack.push(Integer.parseInt(tokens[i])); } } return stack.pop(); } public static void main(String[] args) { System.out.println(new Solution().evalRPN(new String[] { "2", "1", "+", "3", "*" })); System.out.println(new Solution().evalRPN(new String[] { "4", "13", "5", "/", "+" })); }}
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