Problem: Delivery Schedule Validation

You are given a list of strings schedule where each string starts with either:

P - representing a pick-up operation, or
D - representing a delivery operation,
followed by a unique order ID.
Your task is to determine if the schedule is valid based on the following rules:

An order can be picked up only once and delivered only once.
An order must be picked up before it is delivered.
An order must not be delivered if it has not been picked up.

Return True if the schedule follows all the rules, otherwise return False.

Example

Input: ["P1", "D1", "P2", "D2", "P3", "D3"]
Output: True
Explanation: Each pick-up operation precedes its
corresponding delivery operation.

Input: ["P1", "P1", "D1", "D2"]
Output: False
Explanation: Order 1 is picked up more than once.

Input: ["P1", "D1", "D2", "P2"]
Output: False
Explanation: Order 2 is delivered before it is picked up.

Input: ["P132", "D132"]
Output: True
Explanation: Order 1 is picked up and then delivered correctly.

HelloWorld('print')