大话数据结构——使用栈实现简单的四则运算

Posted 2020-10-09

最近在读《大话数据结构》，里面有个例子是使用栈实现四则运算，现在我用java把这个功能实现试试

代码如下：

package com.datastruct;

import java.util.ArrayList;
import java.util.Stack;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class StackPractice {

	private static ArrayList<String> symbolList;
	static{
		symbolList = new ArrayList<>();
		symbolList.add(SymbolEnum.ADD.getVal());
		symbolList.add(SymbolEnum.SUBTRACT.getVal());
		symbolList.add(SymbolEnum.MULTIPLY.getVal());
		symbolList.add(SymbolEnum.DIVIDE.getVal());
		symbolList.add(SymbolEnum.LEFT_BRACKET.getVal());
		symbolList.add(SymbolEnum.RIGHT_BRACKET.getVal());
	}
	
	public static void main(String[] args) {
		String exp = "9+(3-1)*3+10/2";
		try {
			String exp2 = convertExp(exp);
			System.out.println(cal(exp2));
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
	}
	
	private static boolean isNumeric(String str){ 
		   Pattern pattern = Pattern.compile("[0-9]*"); 
		   Matcher isNum = pattern.matcher(str);
		   if( !isNum.matches() ){
		       return false; 
		   } 
		   return true; 
		}

	
	/**
	 * 计算后缀表达式值
	 * @return
	 */
	private static int cal(String exp){
		String[] expArr = exp.split(" ");
		Stack<String> calStack = new Stack<String>();
		for(int i=0;i<expArr.length;i++){
			String tmp = expArr[i];
			if(isNumeric(tmp)){//数字入栈
				calStack.push(tmp);
			}else{
				int num1 = Integer.parseInt(calStack.pop());//栈顶弹出1
				int num2 = Integer.parseInt(calStack.pop());//栈顶弹出2
				switch (tmp) {
				case "-":
					calStack.push(num2 - num1 + "");
					break;
				case "+":
					calStack.push(num2 + num1 + "");
					break;
				case "*":
					calStack.push(num2 * num1 + "");
					break;
				case "/":
					calStack.push(num2 / num1 + "");
					break;
				default:
					break;
				}
			}
		}
		return Integer.parseInt(calStack.pop());
	}
	
	/**
	 * 中缀表达式转为后缀表达式
	 * @return
	 */
	private static String convertExp(String exp) throws Exception{
		//字符串转数组
		char[] expArr = exp.toCharArray();
		StringBuffer sb = new StringBuffer();
		Stack<String> symbolStack = new Stack<>();
		boolean isLastCharNumberFlag = true;
		for(int i = 0;i<expArr.length;i++){
			char curChar = expArr[i];
			//若为数字添加 若为符号则放入栈计算
			if(Character.isDigit(curChar)){
				if(isLastCharNumberFlag){
					sb.append(curChar);
				}else{
					sb.append(" ").append(curChar);
				}
				
				isLastCharNumberFlag = true;
			}else if(symbolList.contains(String.valueOf(curChar))){//符号
				//
				String symbol = String.valueOf(curChar);
				if(SymbolEnum.RIGHT_BRACKET.getVal().equals(symbol) 
						)
				{
					
					//如果是右括号,得匹配左括号 
					String symbolStr = symbolStack.pop();
					while(!SymbolEnum.LEFT_BRACKET.getVal().equals(symbolStr)){
						sb.append(" ").append(symbolStr);
						symbolStr = symbolStack.pop();
					}
					isLastCharNumberFlag = false;
					continue;
				}else if(SymbolEnum.DIVIDE.getVal().equals(symbol) ||
						SymbolEnum.MULTIPLY.getVal().equals(symbol)	){
				}else{
					if(symbolStack.isEmpty()){
						symbolStack.push(symbol);
						continue;
					}
					String symbolStr = symbolStack.peek();
					if(SymbolEnum.DIVIDE.getVal().equals(symbolStr) ||
							SymbolEnum.MULTIPLY.getVal().equals(symbolStr)){
						while(!symbolStack.isEmpty()){
							sb.append(" ").append(symbolStack.pop());
						}
						symbolStack.push(symbol);
						isLastCharNumberFlag = false;
						continue;
					}
				}
				symbolStack.push(symbol);
				isLastCharNumberFlag = false;
			}
		}
		while(!symbolStack.isEmpty()){
			sb.append(" ").append(symbolStack.pop());
		}
		return sb.toString();
	}
	
	
	/**
	 * 运算符枚举
	 * @author Administrator
	 *
	 */
	enum SymbolEnum{
		ADD("+"), SUBTRACT("-"),MULTIPLY("*"),DIVIDE("/"),LEFT_BRACKET("("),RIGHT_BRACKET(")");
		private String val;
		
		public String getVal(){
			return val;
		}
		
		private SymbolEnum(String val){
			this.val = val;
		}
	}
}

　　程序暂时有些bug,如表达式合法性没有校验,不能输入多个括号等