hello-algo/en/docs/chapter_stack_and_queue/exercises.md

Exercises

Check your understanding

Which element comes out first in a stack or queue?

Start with an empty stack S and an empty queue Q. Perform the same operations on each:

Step 1: Add A. Step 2: Add B. Step 3: Remove and record one element. Step 4: Add C. Step 5: Keep removing and recording elements until the container is empty.

Write the order in which elements are removed from S and from Q. Explain the difference using last in, first out or first in, first out.

??? success "Reference answer"

The removal order for stack `S` is `B, C, A`. After adding `A, B`, we first pop `B`, the most recently added element, and then add `C`.
We then pop `C, A` in order. This is last in, first out.

The removal order for queue `Q` is `A, B, C`. After adding `A, B`, we first remove `A`, the earliest element added.
We then add `C` and remove `B, C` in order. This is first in, first out.

What happens when the rear passes the end of the array?

A queue uses a circular array of length 5, with indices 0-4. Currently, front = 3 and size = 2, with queue elements A, B at indices 3 and 4, respectively.

1. When enqueueing C, at which index should C be placed? What is size afterward?
2. If we then dequeue once, which element is removed? What are the new values of front and size?
3. What is the logical order from front to rear now? Does dequeuing require moving the other array elements? Why?

??? success "Reference answer"

1. The new element's position is
    `(front + size) % 5 = (3 + 2) % 5 = 0`,
    so `C` goes at index 0. After enqueueing, `size = 3`.

2. Dequeueing removes the current front element, `A`. The new front index is
    `(3 + 1) % 5 = 4`, so `front = 4` and `size = 2`.

3. The valid elements are logically ordered as `B, C`, with `B` at index 4 and `C` at index 0.
    Dequeueing only advances `front` and updates `size`. The circular array uses the remainder operation to wrap indices back to the beginning,
    so the other elements do not need to shift forward together.

Operations at both ends of a deque

Here, push_first adds at the front, push_last adds at the rear, pop_first removes from the front, and pop_last removes from the rear.

Starting with an empty deque deq, perform:

1. push_last(A)
2. push_last(B)
3. push_first(C)
4. pop_last()
5. push_last(D)
6. pop_first()

1. Which elements are removed by the two pop operations?
2. Which elements remain, from front to rear, after all the operations?
3. Examine these six steps. Can a queue that allows additions only at the rear and removals only at the front perform them all? If not, identify the unsupported operations. Then explain whether a deque can perform them all, and why.

??? success "Reference answer"

After the first three steps, the deque contains `[C, A, B]` from front to rear.

<!-- numbered-subquestions -->

1. `pop_last()` removes `B`. After adding `D`, the deque is `[C, A, D]`.
    Then `pop_first()` removes `C`.

2. The remaining elements are `[A, D]`.

3. A queue that allows additions only at the rear and removals only at the front cannot perform every operation:
    step 3, `push_first(C)`, adds at the front, and step 4, `pop_last()`, removes from the rear. Neither operation is supported by that queue.
    A deque supports additions and removals at both ends, so it can perform all six steps.

Programming exercises

Checking a bracket sequence

Given a string s containing only the three types of brackets (), [], and {}, use a stack to determine whether it is valid.

A valid sequence must satisfy both conditions: each closing bracket matches the type of the most recent unmatched opening bracket, and no unmatched opening brackets remain at the end. Return a Boolean result.

??? tip "Hints"

1. You can map each closing bracket to its matching opening bracket.
2. When you encounter a closing bracket, first check whether the stack is empty, then check whether its top matches.
3. The stack must also be empty when the scan finishes.

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