DSA · Theory

K-way Merge Sort

← All stacks

Theory

562/810

K-way Merge Sort

K-way Merge Sort (alt) is a DSA idea you prove with names A→Z, not with a slogan.

K-way Merge Sort (alt) on the board — n log n vs n². Then say what names A→Z looks like after.

K-way Merge Sort (alt) exists so Vikram can keep names A→Z correct in the roll list.

K-way Merge Sort (alt) shows up in the roll list. Name names A→Z, not “a real-world scenario”.

Don’t do this with K-way Merge Sort (alt): unstable sort when equal keys matter. Interviewers spot it in ten seconds.

After K-way Merge Sort (alt), Vikram should still remember unstable sort when equal keys matter.

One breath for K-way Merge Sort (alt), then names A→Z, then unstable sort when equal keys matter. Sit down.

Exam tip

Board: names A→Z. Dry-run K-way Merge Sort (alt). Name the trap: unstable sort when equal keys matter.

Example

def merge_sort(arr):
    if len(arr) <= 1:
        return arr
    mid = len(arr) // 2
    left = merge_sort(arr[:mid])
    right = merge_sort(arr[mid:])
    out, i, j = [], 0, 0
    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            out.append(left[i]); i += 1
        else:
            out.append(right[j]); j += 1
    return out + left[i:] + right[j:]

print(merge_sort([5, 1, 4, 2]))

K-way Merge Sort (alt) — merge sort divides the array, sorts halves, then merges — O(n log n).

Short notes

DefK-way Merge Sort (alt) — Vikram uses it for names A→Z in the roll list.
RuleK-way Merge Sort (alt) → n log n vs n².
RememberK-way Merge Sort (alt) + a dry-run table (the roll list).
UseK-way Merge Sort (alt) in the roll list (names A→Z).
TrapK-way Merge Sort (alt) — unstable sort when equal keys matter.
ExK-way Merge Sort (alt) → names A→Z.

def merge_sort(arr): if len(arr) <= 1: return arr mid = len(arr) // 2 left = merge_sort(arr[:mid]) right = merge_sort(arr[mid:]) out, i, j = [], 0, 0 while i < len(left) and j < len(right): if left[i] <= right[j]: out.append(left[i]); i += 1 else: out.append(right[j]); j += 1 return out + left[i:] + right[j:] print(merge_sort([5, 1, 4, 2]))

K-way Merge Sort

Example

Short notes

Questions

K-way Merge Sort

Example

Short notes

Questions