**Why Three-Way Partition:** Sort 0s, 1s, 2s in one pass—O(n), O(1). Foundation for 3-way quicksort (Dijkstra's). Used in routing (low/med/high priority), bucketing. **Invariant:** [0..low)=0, [low..mid)=1, [high..n)=2. mid sweeps; swap 0 with low, 2 with high; 1 stays....
This hard-level Python/Coding question appears frequently in data engineering interviews at companies like Microsoft. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (partition) will help you answer variations of this question confidently.
This is a senior-level question that tests architectural thinking. Lead with the high-level design, then drill into specifics. Discuss trade-offs explicitly - there is rarely one correct answer. Show awareness of scale, fault tolerance, and operational complexity.
Why Three-Way Partition: Sort 0s, 1s, 2s in one pass—O(n), O(1). Foundation for 3-way quicksort (Dijkstra's). Used in routing (low/med/high priority), bucketing.
Invariant: [0..low)=0, [low..mid)=1, [high..n)=2. mid sweeps; swap 0 with low, 2 with high; 1 stays.
Extensions: K-way partition needs different approach. With duplicates, stability is lost—acceptable for sort. In streaming: classify into 3 buckets without full sort.
def sort_012(arr):
lo, mid, hi = 0, 0, len(arr)-1
while mid <= hi:
if arr[mid] == 0:
arr[lo], arr[mid] = arr[mid], arr[lo]
lo += 1; mid += 1
elif arr[mid] == 1: mid += 1
else:
arr[mid], arr[hi] = arr[hi], arr[mid]
hi -= 1
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Analyze My Answer — FreeAccording to DataEngPrep.tech, this is one of the most frequently asked Python/Coding interview questions, reported at 1 company. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.