dsa · easy

Stack from Queues

Implement a last-in-first-out stack. Internally you may only use FIFO queues (append at the back, pop from the front). Construct with MyStack(). Then support push(x), pop(), top(), and empty().

pop() and top() are never called on an empty stack except through empty() itself.

Fill in MyStack; the starter walks ops / args for you.

Methods

• MyStack() create an empty stack
• push(x) push x onto the stack; returns nothing
• pop() remove and return the top value
• top() return the top value without removing it
• empty() true if the stack holds nothing

Example

`` MyStack() empty push(1) [1] 1 is top push(2) [1, 2] 2 is top top() → 2 stack unchanged pop() → 2 [1] remains empty() → false ``

Fill in the MyStack class. The starter already walks ops / args and calls your methods — leave the driver at the bottom as-is. Constructors contribute null; booleans print as true / false.

Constraints

Calls <= 100 1 <= x <= 10^9

Examples

Example 1

Input:
["MyStack","push","push","top","pop","empty"]
[[],[1],[2],[],[],[]]

Expected:
[null,null,null,2,2,false]

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