How should I practice String Deque with Constant-Time Operations?

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

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Problem · Design

String Deque with Constant-Time Operations

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Problem statement

Implement a deque of strings with a doubly linked list and process operations in order.

push_front value and push_back value insert a value.
pop_front and pop_back remove and return one end, or return EMPTY.
front and back return one end without removal, or EMPTY.
size returns the decimal size.

Return one string for every pop, read, or size command. Every operation must run in O(1) time.

Function

processStringDeque(operations: String[]) → String[]

Examples

Example 1

operations = ["push_back a","push_front b","front","back","size","pop_front","pop_back","pop_back"]return = ["b","a","2","b","a","EMPTY"]

Values are read and removed from both ends; the final pop sees an empty deque.

Example 2

operations = ["front","push_back x","pop_back","size"]return = ["EMPTY","x","0"]

An empty read is reported, then x is inserted and removed.

Constraints

1 <= operations.length <= 200000
Inserted values are nonempty strings without spaces and are never the reserved word EMPTY.
Every operation uses one of the documented forms.