How should I practice Constant-Time Minimum Stack?

Start by reading the full statement, solve the visible examples, then compare the pattern against related LinkedIn phone screen problems in FastPrep.

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Constant-Time Minimum Stack

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Problem statement

Implement a stack that supports pushing, popping, reading the top value, and retrieving the minimum value, each in O(1) time.

For this exercise, process a finite list of valid operation strings in order, starting with an empty stack:

PUSH x: place integer x on top.
POP: remove the top value and produce no output.
TOP: append the top value to the result without modifying the stack.
MIN: append the smallest value currently in the stack to the result without modifying the stack.

Return the integer results of only TOP and MIN, in operation order. Preserve repeated values as separate stack entries. Every POP, TOP, and MIN is guaranteed to occur while the stack is nonempty. If there are no query operations, return an empty array.

Function

runMinStack(operations: String[]) → int[]

Examples

Example 1

operations = ["PUSH -2","PUSH 0","PUSH -3","MIN","POP","TOP","MIN"]return = [-3,0,-2]

The first minimum is -3. After popping it, the top is 0 and the minimum among the remaining values is -2.

Example 2

operations = []return = []

No operations produce no query results.

Example 3

operations = ["PUSH 5","PUSH 5","MIN","POP","MIN"]return = [5,5]

Popping one occurrence of 5 leaves the other occurrence, so both minimum queries return 5.

Constraints

For this exercise, assume 0 <= operations.length <= 1000.
Every operation is exactly PUSH x, POP, TOP, or MIN; PUSH uses one space and a base-10 integer without a leading plus sign.
-10^9 <= x <= 10^9.
Every non-push operation has a nonempty stack. The whole sequence must run in O(m) time for m operations, with O(1) work per operation after allocating storage.