150. Evaluate Reverse Polish Notation

最新推荐文章于 2021-03-20 22:13:41 发布

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最新推荐文章于 2021-03-20 22:13:41 发布

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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

import java.util.Stack;

public class Solution {
    public int evalRPN(String[] tokens) {
        int rst = 0;
        Stack<String> stack = new Stack<String>();
        for(String s : tokens) {
        	if("+".equals(s)) {
        		int a = Integer.parseInt(stack.pop()), b = Integer.parseInt(stack.pop());
        		rst = a + b;
        		stack.push(rst + "");
        	} else if("-".equals(s)) {
        		int a = Integer.parseInt(stack.pop()), b = Integer.parseInt(stack.pop());
        		rst = b - a;
        		stack.push(rst + "");
        	} else if("*".equals(s)) {
        		int a = Integer.parseInt(stack.pop()), b = Integer.parseInt(stack.pop());
        		rst = a * b;
        		stack.push(rst + "");
        	} else if("/".equals(s)) {
        		int a = Integer.parseInt(stack.pop()), b = Integer.parseInt(stack.pop());
        		rst = b / a;
        		stack.push(rst + "");
        	} else {
        	    rst = Integer.parseInt(s);
        		stack.push(s);
        	}
        }
        
        return rst;
    }
}