利用栈实现简单算术表达式求值

代码利用栈来实现算术表达式的求值功能，仅用于举例说明栈的用例，没有考虑运算符的优先级，并且要求表达式的各个部分必须用空格分隔：

package com.mycode.algorithms.stack;

import java.util.HashMap;
import java.util.Map;
import java.util.Stack;

public class Evaluate {
	static Stack<String> ops = new Stack<String>();
	static Stack<Double> params = new Stack<Double>();
	
	static Map<String,String> VALIDATE_OPS = new HashMap<String,String>();
	static {
		VALIDATE_OPS.put("+", "+");
		VALIDATE_OPS.put("-", "-");
		VALIDATE_OPS.put("*", "*");
		VALIDATE_OPS.put("/", "/");
	}

	public static void main(String[] args) {
		System.out.println(calcute("( 5 + ( 3 * ( 4 + 5 ) ) + 100 )"));
	}
	
	public static void clear(){
		ops.clear();
		params.clear();
	}
	
	public static boolean isOps(String symbol){
		return VALIDATE_OPS.get(symbol)!=null;
	}
	
	public static Double calcuteValue(String symbol,Double v1,Double v2){
		String op = VALIDATE_OPS.get(symbol);
		if(op.equals("+")){
			return v1 + v2;
		}else if(op.equals("-")){
			return v1 -v2;
		}else if(op.equals("*")){
			return v1 * v2;
		}else if(op.equals("/")){
			return v1 / v2;
		}
		throw new RuntimeException("not supported operation:"+op);
	}
	
	
	public static Double calcute(String expression){
		clear();
		for(String symbol : expression.split(" ")){
			if(symbol.trim().equals("") || symbol.equals("(")) continue;
			if(isOps(symbol)){
				ops.add(symbol);
			}else if(symbol.equals(")")){
				params.add(calcuteValue(ops.pop(), params.pop(), params.pop()));
			}else{
				params.add(Double.valueOf(symbol));
			}
			System.out.println(params);
			System.out.println(ops);
			System.out.println("====================");
		}
		if(!ops.isEmpty()){
			params.add(calcuteValue(ops.pop(), params.pop(), params.pop()));
		}
		return params.pop();
	}
}

运行结果：

[5.0]
[]
====================
[5.0]
[+]
====================
[5.0, 3.0]
[+]
====================
[5.0, 3.0]
[+, *]
====================
[5.0, 3.0, 4.0]
[+, *]
====================
[5.0, 3.0, 4.0]
[+, *, +]
====================
[5.0, 3.0, 4.0, 5.0]
[+, *, +]
====================
[5.0, 3.0, 9.0]
[+, *]
====================
[5.0, 27.0]
[+]
====================
[5.0, 27.0]
[+, +]
====================
[5.0, 27.0, 100.0]
[+, +]
====================
[5.0, 127.0]
[+]
====================
132.0