Question

I want to know why we always use Sorting algorithm like (Insertion Sort or Merge Sort,…) just for lists and arrays? And why we do not use these algorithms for stack or queue?

Answer 1

Stacks and queues are abstract data types that have their own sense of order, i.e. LIFO (Last In First Out) for stacks and FIFO (First In First Out) for queues. As such, it does not make sense to take a queue/stack and reorder their elements around.

Wikipedia references

• Stack (data structure)
• Queue (data structure)

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On Stack vs Vector

You may notice that in Java, java.util.Stackextendsjava.util.Vector, and since it makes sense to sort a Vector, perhaps it also makes sense to sort a Stack. This is not the case however; the fact that Stack extends Vector is in fact a design blunder. A stack is NOT a vector.

Related questions

• Java Stack class inherit Vector Class

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On using Collections.sort on java.util.Stack

Despite the fact that it doesn’t make sense to use, say, quicksort on a stack, you CAN actually use Collections.sort on a java.util.Stack. Why? Because, by virtue of design error (this can’t be emphasized enough!), a java.util.Stack is a java.util.Vector, which implements java.util.List, and you certainly can sort a List. Here’s an example:

    Stack<Integer> stack = new Stack<Integer>();
    stack.push(1);
    stack.push(3);
    stack.push(5);
    stack.push(2);
    stack.push(4);

    Collections.sort(stack); // by virtue of design error!!!

    System.out.println(stack); // prints "[1, 2, 3, 4, 5]"
    while (!stack.isEmpty()) {
        System.out.println(stack.pop());
    } // prints "5", "4", "3", "2", "1"

Note that the elements are printed in descending order: this is because of how java.util.Stack is implemented. It pushes to and pops from the end of the Vector. You don’t need to know this; you shouldn’t have known this; but these are the facts.

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On using an appropriate data structure

Depending on what it is that you’re trying to accomplish, a TreeSet may be the appropriate data structure. It is a Set, so it does not permit duplicate elements.

    NavigableSet<Integer> nums = new TreeSet<Integer>();
    nums.add(5);
    nums.add(3);
    nums.add(1);
    nums.add(2);
    nums.add(6);

    System.out.println(nums.pollFirst()); // prints "1"
    System.out.println(nums.pollFirst()); // prints "2"
    nums.add(4);
    System.out.println(nums.pollFirst()); // prints "3"
    System.out.println(nums.pollFirst()); // prints "4"