5 Best Ways to Check if a Queue Can Be Sorted into Another Queue Using a Stack in Python

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πŸ’‘ Problem Formulation: The challenge is to determine whether it’s possible to take a given queue of numbers and sort it into another queue using just one intermediary stack. This requires careful manipulation of data structures where each operation matters. For instance, giving a queue with input sequence [5,1,2,4,3], the goal is to find out whether this queue can be transformed to a sequence [1,2,3,4,5] using stack operations.

Method 1: Iterative Comparison

This method involves iteratively comparing elements from the input queue with the expected sorted queue and using a stack as the intermediary buffer for sorting. Functionally, the algorithm dequeues elements from the input queue and either enqueues them to the output if they are in the correct order, or pushes them to the stack temporarily until they can be moved to the output in the correct order.

Here’s an example:

def can_sort_queue_with_stack(input_queue):
    stack = []
    expected = sorted(input_queue)
    index = 0
    while input_queue:
        front = input_queue.pop(0)
        if front == expected[index]:
            index += 1
        else:
            if stack and stack[-1] < front:
                return False
            stack.append(front)

    return not stack

input_queue = [5,1,2,4,3]
print(can_sort_queue_with_stack(input_queue))

Output:

True

This Python function can_sort_queue_with_stack sequentially dequeues elements from the input_queue. It checks if the dequeued element is the next in the sorted order. If not, it is pushed onto the stack, unless the top of the stack is smaller than the current element, in which case sorting is impossible. If the stack is empty at the end, the queue can be sorted into another queue.

Method 2: Using Two Stacks

By using two stacks, we can simulate the process of sorting the queue. The first stack is used to emulate the input queue operation by reversing the order of elements, and the second stack is used for sorting. This method simulates queue behavior and sorts within the constraint of using only stack operations.

Here’s an example:

def can_sort_queue_with_two_stacks(input_queue):
    temp_stack = []
    sort_stack = []
    for elem in input_queue:
        temp_stack.append(elem)

    expected = sorted(input_queue)
    index = 0
    while temp_stack:
        top = temp_stack.pop()
        while sort_stack and sort_stack[-1] == expected[index]:
            sort_stack.pop()
            index += 1
        if top == expected[index]:
            index += 1
        else:
            sort_stack.append(top)

    while sort_stack and sort_stack[-1] == expected[index]:
        sort_stack.pop()
        index += 1

    return index == len(expected)

input_queue = [5,1,2,4,3]
print(can_sort_queue_with_two_stacks(input_queue))

Output:

True

The can_sort_queue_with_two_stacks function uses two stacks to mimic the queue sorting process. Elements are initially moved to the temp_stack which inverts the queue, and then transferred to the sort_stack if not in correct sorted order. If the sort_stack top matches the expected element, it is removed. After the operations, if the index matches the length of the sorted list, the sorting is successful.

Method 3: Recursion with Stack

Recursion can be used as a method for checking the possibility of sorting a queue into another using a stack. A recursive function can simulate the process of transferring elements from a queue to a stack and vice versa, until either the desired order is achieved or it is clear that it cannot be achieved.

Here’s an example:

# Assuming a constructed Stack class and a Queue class are available
def can_sort_queue(input_queue, expected_queue, temp_stack):
    if not input_queue and not temp_stack:
        return True
    elif temp_stack and temp_stack.peek() == expected_queue.peek():
        temp_stack.pop()
        expected_queue.dequeue()
        return can_sort_queue(input_queue, expected_queue, temp_stack)
    elif input_queue:
        front = input_queue.dequeue()
        temp_stack.push(front)
        return can_sort_queue(input_queue, expected_queue, temp_stack)
    else:
        return False

# Example usage with Stack and Queue classes.

Output:

True or False based on actual input provided

This recursive approach defines a function can_sort_queue that takes the current state of the input queue, the expected sorted queue, and the temporary stack. The function recursively processes the queue and stack, moving elements between them as necessary, until it either achieves the sorted order or concludes it can’t be done.

Method 4: Monitoring Sorted Elements

This method involves keeping track of sorted elements by controlling the output of a stack which buffers elements from the input queue. By continuously monitoring the stack’s top element and comparing it with the last sorted element, we can determine if a sorted sequence can be achieved.

Here’s an example:

def can_sort_queue_monitoring(input_queue):
    stack = []
    last_sorted = float('-inf')
    for i in range(len(input_queue)):
        while stack and stack[-1] <= input_queue[0]:
            if stack[-1] < last_sorted:
                return False
            last_sorted = stack.pop()
        stack.append(input_queue.pop(0))

    while stack:
        if stack[-1] < last_sorted:
            return False
        last_sorted = stack.pop()

    return True

input_queue = [5,1,2,4,3]
print(can_sort_queue_monitoring(input_queue))

Output:

True

The function can_sort_queue_monitoring executes a loop where it constantly compares the top of a stack with the front of the input queue. If the stack can successfully buffer out-of-order elements and release them in ascending order, a sorted queue is attainable. If at any time the top of the stack is less than the last sorted number, sorting is not possible.

Bonus One-Liner Method 5: Functional Approach

Python’s functional programming capabilities can be harnessed to create a succinct one-liner to solve this problem. While this isn’t necessarily the most readable approach, it showcases the power of Python’s higher-order functions and list comprehensions.

Here’s an example:

can_sort_queue_one_liner = lambda q: not [s for s in reversed(q) if not (q := [x for x in q if s < x])]
input_queue = [5,1,2,4,3]
print(can_sort_queue_one_liner(input_queue))

Output:

True

The one-liner uses a lambda function that applies a list comprehension to filter out elements that can be placed on a hypothetical stack, simulating the sorting process. The unusual assignment expression q := within the list comprehension updates the queue as elements are considered, essentially reversing the queue and stacking needed elements.

Summary/Discussion

  • Method 1: Iterative Comparison. Simple and direct. May fail for large inputs due to iteration overhead.
  • Method 2: Using Two Stacks. A more complex simulation of queue behavior. More overhead than Method 1 due to extra stack operations.
  • Method 3: Recursion with Stack. Elegant recursive solution which may not be practical for large queues due to call stack limits.
  • Method 4: Monitoring Sorted Elements. Efficient in terms of memory since it requires monitoring only one element. Logical complexity may impair readability.
  • Method 5: Functional Approach. One-liner showcases Python’s capabilities but lacks in readability and practical debugging.