谁能告诉我我在做什么错？ -堆栈

Question

I am required to write a method, compress to do the following;

The objective of the method compress is to remove all null elements from the stack s1. The remaining (non-null) elements should be kept on s1 in their initial order. The auxiliary stack s2 should be used as a temporary storage for the elements from s1. At the end of the computation of this method, stack s2 should have the same content as at the beginning of the computation. See the method main for an example of the expected behaviour of the method compress.

So far I have;

import net.datastructures.ArrayStack;
import net.datastructures.Stack;

public class Stacks {

public static <E> void compress(Stack<E> S1, Stack<E> S2) {

    int counter = 0;
    while (!S1.isEmpty()) {

    }
    if (S1.top() == null) {
        S1.pop();
    } else if (S1.top() != null) {
        S2.push(S1.pop());

        counter++;
    }

    for (int i = counter; i < counter; i++) {

        S2.push(S1.pop());
    }
}

public static void main(String[] args) {
    // test method compress
    Stack<Integer> S1 = new ArrayStack<Integer>(10);
    S1.push(2);
    S1.push(null);
    S1.push(null);
    S1.push(4);
    S1.push(6);
    S1.push(null);

    Stack<Integer> S2 = new ArrayStack<Integer>(10);
    S2.push(7);
    S2.push(9);

    System.out.println("stack S1: " + S1);
    // prints: "stack S1: [2, null, null, 4, 6, null]"

    System.out.println("stack S2: " + S2);
    // prints: "stack s2: [7, 9]"

    compress(S1, S2);

    System.out.println("stack S1: " + S1);
    // should print: "stack S1: [2, 4, 6]"

    System.out.println("stack S2: " + S2);
    // should print: "stack S2: [7, 9]"
}

}

I can't figure out where I'm going wrong, the code prints the two lines before the compress method and then prints nothing.

Answer 1

while (!S1.isEmpty()) {

}

Right there, you have an infinite loop.

Answer 2

i guess writing the if...else inside your while() as below

while (!S1.isEmpty()) {
if (S1.top() == null) {
    S1.pop();
} else if (S1.top() != null) {
    S2.push(S1.pop());

    counter++;
}

}

and inside your for it must be something like

for (int i = counter; i < counter; i++) {

    S1.push(S2.pop());
}

guess it should work

Answer 3

I spotted two mistakes in your code. One on each of your cycles.

1. Your while loop should be wrapping the conditionals right after it.
2. Your for loop has its increment variable badly assigned.

Correct version should be:

public static <E> void compress(Stack<E> S1, Stack<E> S2) {
    int counter = 0;

    while (!S1.isEmpty()) {
        if (S1.top() == null) {
            S1.pop();
        } else {
            S2.push(S1.pop());
            counter++;
        }
    }

    for (int i = 0; i < counter; i++) {
        S1.push(S2.pop());
    }
}

Edit: An equivalent for loop (maybe you were trying to write this one) may be as follows.

for (int i = counter; i > 0; i--) {
    S1.push(S2.pop());
}

2nd edit: The variables in the for loops were switched (S2 was in S1's place and vice versa).