Stacks and queues are simple ideas with a lot of power. A stack removes the most recent item first. A queue removes the oldest item first. The hard part is not the definition; the hard part is recognizing which behavior the problem needs.
Stack: last in, first out
A stack is useful when the newest item should be handled first. Examples include undo history, matching parentheses, recursion tracing, and depth-first exploration.
Stack<String> history = new Stack<String>();
history.push("home");
history.push("profile");
System.out.println(history.pop());This prints profile because it was added most recently.
Queue: first in, first out
A queue is useful when items should be handled in arrival order. Examples include waiting lines, print jobs, customer requests, and breadth-first exploration.
How to decide
- If the problem says "most recent" or "backtrack," think stack.
- If the problem says "in order received" or "fair waiting line," think queue.
- If the problem explores neighbors level by level, think queue.
- If the problem explores one path deeply before returning, think stack.
Practice idea
Trace the same sequence of values through a stack and a queue. Students quickly see that the operations may look similar, but the removal order changes everything.