数据结构中的栈

Question

我是数据新手 structures.I 尝试输入 5 个字符并使用 pop() 函数反转它。但数组中最后一个索引的值显示为空。谁能解释一下原因吗？

堆叠Class:

package stack;

public class Stack {
    
    int top = 0;
    String stack[] = new String[5];
    
    public void push(String val) {
        
        if(top >= 4) {
            
            System.out.println("Overflow Condition");
            
        }
        
        else {
            
            stack[top] = val;
        
            System.out.println("new value for index"+top+" " +"is"+" "+stack[top]);
            
            top++;
            
        }
        
    }
    
    public void pop() {
        
        String output ;
        
        if(top == -1) {
            
            System.out.println("Underflow Condition");
        }
        
        else {
            
            
            for(int i= top; i >= 0; i--) {
            
            output = stack[top];
            
            System.out.println("Removed Index:"+" "+(top+1)+"is"+ output);
            top--;
            
            }
            
        }
        
    }
    
    public void peek() {
        
        String output;
        
        if(top >= 4) {
            
            System.out.println("Overflow Condition");
        }
        
        else {
            
            output = stack[top];
            
        }
        
    }   
    
}

主程序：

package stack;

import java.util.Scanner;

public class StackHome {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Scanner inp = new Scanner(System.in);
        
        Stack obj = new Stack();
        
        String arr[] = new String[5];
        
        for(int i=0; i<5; i++) {
        
            System.out.println("Value to add for stack");
            String value = inp.next();
            
            arr[i] = value;
            
            obj.push(arr[i]);
        }
        
        obj.pop();
        //obj.peek();
        
    }

}

输入

Value to add for stack
a
new value for index0 is a
Value to add for stack
b
new value for index1 is b
Value to add for stack
c
new value for index2 is c
Value to add for stack
d
new value for index3 is d
Value to add for stack
e

输出

Overflow Condition
Removed Index: 4is null
Removed Index: 3is d
Removed Index: 2is c
Removed Index: 1is b Removed Index: 0is a

Answer 1

我想指出一些更改以简化您的代码。

您不需要在 main() 方法中创建新的 array，因为您已经创建了 array（用作 stack) 在 Stack class.

public static void main(String[] args) {
    Scanner inp = new Scanner(System.in);
    Stack obj = new Stack();
    //this loop is for pushing elements into the stack
    for (int i = 0; i < 5; i++) {
        System.out.println("Value to add for stack");
        String value = inp.next();
        //you only need to call the push() method to push the element into the array created in the Stack class.
        obj.push(value);
    }
    //this loop is for popping the elements from the stack
    for (int i = 0; i < 5; i++) {
        System.out.println("Value to remove from stack");
        //you just need to call the pop() method to pop an element from the stack 
        obj.pop();
    }
    inp.close();
}

您的 pop() 方法不需要 for 循环，因为它的唯一工作是删除元素。您可以运行 main() 中的循环来一个接一个地弹出元素，如我上面所示。所以你的pop()方法可以修改如下：

public String pop() {
    if (top == 0) {
        System.out.println("Underflow Condition");
    }
    else { //no need of for loop here.
            top--;
            String output = stack[top];
            System.out.println("Removed Index:" + " " + (top + 1) + "is" + output);
            return output;
    }
    return null;
}

注意：一旦所有元素都被推入stack，top的值将为5，即length数组，因此您需要先使用 top-- 减少 top 的值，以便 top 指向数组的最后一个索引，然后您可以 return 弹出的元素。

Input/Output如下图：

Value to add for stack
a b c d e
new value for index0 is a
Value to add for stack
new value for index1 is b
Value to add for stack
new value for index2 is c
Value to add for stack
new value for index3 is d
Value to add for stack
new value for index4 is e
Value to remove from stack
Removed Index: 5ise
Value to remove from stack
Removed Index: 4isd
Value to remove from stack
Removed Index: 3isc
Value to remove from stack
Removed Index: 2isb
Value to remove from stack
Removed Index: 1isa

希望这能回答您的问题。

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