Practice Deque Operations

Practice Exercise Beginner Easy Deque Queue

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Problem

Implement run_deque_commands(commands). Commands are ["append-right", value], ["append-left", value], ["pop-left"], or ["pop-right"]. Return one result for every pop; an empty pop returns None.

Starter code

from collections import deque

def run_deque_commands(commands):
    pass

Test cases

mixed-ends

{
  "args": [
    [
      [
        "append-right",
        2
      ],
      [
        "append-left",
        1
      ],
      [
        "pop-right"
      ],
      [
        "pop-left"
      ]
    ]
  ]
}

Expected: [2,1]

empty-removals

{
  "args": [
    [
      [
        "pop-left"
      ],
      [
        "pop-right"
      ]
    ]
  ]
}

Expected: [null,null]

Wizard outline

Step 1: Guard an empty removal
Create persistent deque state and record None for an empty pop-left command. An explicit empty policy prevents IndexError before mutation commands are introduced.
Step 2: Operate on the right end
Append supplied values on the right and remove the newest right-side value safely. Right-side append and pop establish the familiar stack-shaped half of deque behavior.
Step 3: Operate on the left end
Insert values on the left while preserving both right-side operations. appendleft adds the operation that distinguishes deque from a plain list stack.

Footguns and prerequisites

Using list.pop(0) makes repeated left removals linear.
Calling popleft or pop on an empty deque raises IndexError unless guarded.
python specific rapid fire

Reviewed references

Recommended approach and implementation

Process the command stream once with collections.deque and collect only pop results.

Why it works: Each command invokes the matching deque end operation, and guarded removals return exactly the requested value or None when no value exists.

from collections import deque

def run_deque_commands(commands):
    items = deque()
    results = []
    for command in commands:
        operation = command[0]
        if operation == "append-right":
            items.append(command[1])
        elif operation == "append-left":
            items.appendleft(command[1])
        elif operation == "pop-left":
            results.append(items.popleft() if items else None)
        elif operation == "pop-right":
            results.append(items.pop() if items else None)
    return results