如何找到堆栈中的最大值？ （爪哇）

Question

I am preparing for an exam on stacks and I'm practicing on various methods regarding stack. I tried to implement a method that return the highest value in a stack but I'm a little bit confused.

I should remove the stack top of this and push it in a new stack s1 , then compare this.stackTop with s1.stackTop .

This is my code:

        Stack s1 = new Stack();
        
        if(this.isEmpty()) {
            System.out.println("the stack is empty ");
            return NOT_FOUND;
        }
        
        while(!this.isEmpty() && this.stackTop() > s1.stackTop()) {
            s1.push(this.stackTop());
            this.pop(); 
        }
        
        while(!s1.isEmpty()) {
            this.push(s1.stackTop());
            s1.pop();
        }
    }

I'm just confused and don't know how to proceed so any help would be appreciated.

Answer 1

If the stack isn't sorted (which it normally isn't, since it would be very inefficient to sort stacks) you need to run through the entire stack. So you would have to run through every element until the stack is empty, just like when finding the largest number from an array. On a stack though you can't access elements in the middle of the stack, so you have to use s1 to track and "shift" the elements from the main stack to be able to compare them all. After you've found the largest value you need to return all the elements in s1 back to the main stack.

Imagine you have a box. In this box there are books which are all the same size and perfectly fit the box. This means you can't just take out any book from the box. So to find the book with the most pages you need to take out every book and compare it to the current largest book you have found. And since you want to put the books back in order you put them in a second box. Afterwards when you found the longest book take each book out of the temporary box and put it back into the original box.

// Create your temporary stack
Stack s1 = new Stack();
// Initialize the first value with the minimal value
int largest = Integer.MIN_VALUE;

if (this.isEmpty()) {
    System.out.println("the stack is empty ");
    return NOT_FOUND;
}

// Loop through the main stack until it's empty
while (!this.isEmpty()) {
    // Take out the top element
    int current = this.pop();
    // Compare it to the largest that you have found until now
    if (current > largest) {
        // If it's larger remember it
        largest = current;
    }

    // And put it into the temporary stack
    s1.push(current);
}

// Loop through the temporary stack until it's empty
while (!s1.isEmpty()) {
    // Take the top element from the temporary stack and put it onto the main one
    this.push(s1.pop());
}