468. Track Maximum Stack Value

You are given a list of operations to perform on an initially empty stack. The operations are either "PUSH x" (where x is an integer) or "POP". After each operation, record the maximum value currently in the stack. If the stack is empty, record -1. Return the list of recorded maximums.

Example:
Input: ops = ["PUSH 3", "PUSH 5", "POP", "POP"]
Output: [3, 5, 3, -1]

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