How should I practice Lazy K-Way Union Iterator?

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

Is Lazy K-Way Union Iterator associated with MongoDB?

FastPrep tags this as a MongoDB phone screen coding practice problem when the problem catalog has that company and stage context.

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Lazy K-Way Union Iterator

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Problem statement

You are given sortedIterators, where each row represents the values produced by one finite iterator in nondecreasing order, and an ordered list of iterator operations.

Process the operations using one lazy union iterator over all input iterators:

has_next returns whether at least one input iterator still has an unconsumed value. It must not consume any value.
next returns the smallest currently available value as a base-10 string and advances only the input iterator that supplied that value.
If several iterators currently expose the same minimum value, use the iterator with the smallest input index. Equal values are separate elements and must all be returned.
If next is called after every input iterator is exhausted, return ERROR_EXHAUSTED without changing state.

Return one string result for every operation, in order. A has_next result is true or false.

The iterator must be lazy: do not materialize the complete merged sequence. Keep only the state needed to identify the next available value and advance its source iterator.

Function

runUnionIterator(sortedIterators: int[][], operations: String[]) → String[]

Examples

Example 1

sortedIterators = [[1,4,7],[2,3,8]]operations = ["has_next","next","next","has_next","next"]return = ["true","1","2","true","3"]

The first query observes that values remain. The next three consumed values are 1, 2, and 3; each comes from the iterator whose current value is globally smallest.

Example 2

sortedIterators = [[1,1,9],[1,2],[]]operations = ["next","next","next","next","next","next","has_next","next"]return = ["1","1","1","2","9","ERROR_EXHAUSTED","false","ERROR_EXHAUSTED"]

All three copies of 1 are retained. After 9 is consumed, repeated exhausted calls do not change state, and has_next remains false.

Constraints

0 <= sortedIterators.length <= 200000.
The total number of values across all rows is at most 200000.
Each row is sorted in nondecreasing order.
Every value fits in a signed 32-bit integer.
0 <= operations.length <= 200000.
Every operation is either has_next or next.
The solution must not materialize the complete merged sequence.