數據結構與算法13-棧和隊列-解析算法表達式

 算法表達式有前綴算法表達式，中綴算法表達式，後綴算法表達式。

計算機更適合用後綴算法表達式，並用棧來實現

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;


class StackX3 {
	private int maxSize;
	private int[] stackArray;
	private int top;
	public StackX3(int size)
	{
		maxSize = size;
		stackArray = new int[maxSize];
		top = -1;
	}
	public void push(int j)
	{
		stackArray[++top] = j;
	}
	public int pop() {
		return stackArray[top--];
	}
	public int peek()
	{
		return stackArray[top];
	}
	public boolean isEmpty()
	{
		return (top == -1);
	}
	public boolean isFull()
	{
		return (top == maxSize -1);
	}
	public int size()
	{
		return top+1;
	}
	public int peekN(int n)
	{
		return stackArray[n];
	}
	public void displayStack(String s)
	{
		System.out.print(s);
		System.out.println("Stack (bottom-->top): ");
		for(int j=0;j<size();j++)
		{
			System.out.print(peekN(j));
			System.out.print(" ");
		}
		System.out.println(" ");
	}
}
class ParsePost{
	private StackX3 theStack;
	private String input;
	public ParsePost(String s)
	{
		input = s;
	}
	public int doParse()
	{
		theStack = new StackX3(20);
		char ch;
		int j;
		int num1,num2,interAns;
		for(j=0;j<input.length();j++)
		{
			ch = input.charAt(j);
			theStack.displayStack(""+ch+" ");
			if(ch >= '0'&&ch <= '9')
				theStack.push((int)(ch-'0'));
			else {
				num2 = theStack.pop();
				num1 = theStack.pop();
				switch (ch) {
				
				case '+':
					interAns = num1+num2;
					break;
				case '-':
					interAns = num1-num2;
					break;
				case '*':
					interAns = num1*num2;
					break;
				case '/':
					interAns = num1/num2;
					break;
				default:
					interAns = 0;
				}
				theStack.push(interAns);
			}
		}
		interAns = theStack.pop();
		return interAns;
	}
}
class PostfixApp
{
	public static void main(String[] args) throws IOException
	{
		String input;
		int output;
		while(true)
		{
			System.out.println("enter postfix: ");
			System.out.flush();
			input = getString();
			if(input.equals(" "))
				break;
			ParsePost aParser = new ParsePost(input);
			output = aParser.doParse();
			System.out.println("evaluates to" + output);
		}
	}
	public static String getString() throws IOException
	{
		InputStreamReader isr = new InputStreamReader(System.in);
		BufferedReader br = new BufferedReader(isr);
		String s = br.readLine();
		return s;
	}
}

enter postfix: 
444+*315+/-
4 Stack (bottom-->top): 
 
4 Stack (bottom-->top): 
4  
4 Stack (bottom-->top): 
4 4  
+ Stack (bottom-->top): 
4 4 4  
* Stack (bottom-->top): 
4 8  
3 Stack (bottom-->top): 
32  
1 Stack (bottom-->top): 
32 3  
5 Stack (bottom-->top): 
32 3 1  
+ Stack (bottom-->top): 
32 3 1 5  
/ Stack (bottom-->top): 
32 3 6  
- Stack (bottom-->top): 
32 0  
evaluates to32
enter postfix: