by, Subhajit Gorai
14/11/2025

Quick Sort

Quick Sort is a divide-and-conquer sorting algorithm that works by selecting a pivot element, partitioning the array into elements smaller and larger than the pivot, and then recursively sorting the partitions. It is efficient on average with O(nlog⁡n)O(n\log n) time complexity.

Quick Look:

  1. Time Complexity: Best/Average: O(nlog⁡n)O(n \log n) Worst: O(n2)O(n^2) (when pivot is always min/max)
  2. Space Complexity: O(log⁡n)O(\log n) (recursion stack)
  3. Stability: Unstable, consider an array of same elements, i pick the first one as partition. now it will be joined at the end. because, [<=partition]+[partition]+[>partition]. read more

Algorithm (Recursive Partitioning):

  1. Pick a pivot.
  2. Partition array: elements < pivot on left, > pivot on right.
  3. Recursively sort left and right subarrays.

code:

python
def quick_sort(arr: list[int]) -> list[int]:
    if len(arr) <= 1:
        return arr

    pivot = arr[0]
    smaller_eq = [x for x in arr[1:] if x <= pivot]
    larger  = [x for x in arr[1:] if x > pivot]

    return quick_sort(smaller_eq) + [pivot] + quick_sort(larger)

inplace version:

python
def partition(arr, low, high):
    pi = high
    i = low - 1 # i points to the last index that was smaller than the pivot element

    for j in range(low, high):
        if arr[j] <= arr[pi]:
            i += 1
            arr[i], arr[j] = arr[j], arr[i] # swap arr[i], arr[j]
            # why needed? dry run: [pi-x, pi-y, pi+a, pi-z, pi]

    arr[i+1], arr[pi] = arr[pi], arr[i+1] # swapping i+1 with pivot,
    # i <- last index that was smaller than pivot, so i+1 -> pivot's suitable position
    return i+1 # return pivots index

def quicksort(arr, low, high):
    if low < high:
        pi = partition(arr, low, high)
        quicksort(arr, low, pi-1)
        quicksort(arr, pi+1, high)

Visualisation, Sorting [7, 3, 5, 2, 9]:

sh
QUICK SORT VISUALIZATION
=========================

Initial array:
+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |   <-- value
+---+---+---+---+---+
  0   1   2   3   4     <-- index


========================================
PARTITION 1: Full array [7,3,5,2,9]
========================================
Range: left=0, right=4
Pivot: Choose last element = 9 (index 4)

+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |  ← pivot = 9
+---+---+---+---+---+
  i=0             ^
               pivot

Goal: Move elements < 9 to left, >= 9 to right

i = -1 (tracks position for next small element)
j = 0  (scans through array)

j=0: arr[0]=7 < 9 → increment i to 0, swap arr[0] with arr[0]
+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |
+---+---+---+---+---+
  i=0             ^
               pivot

j=1: arr[1]=3 < 9 → increment i to 1, swap arr[1] with arr[1]
+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |
+---+---+---+---+---+
      i=1         ^
               pivot

j=2: arr[2]=5 < 9 → increment i to 2, swap arr[2] with arr[2]
+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |
+---+---+---+---+---+
          i=2     ^
               pivot

j=3: arr[3]=2 < 9 → increment i to 3, swap arr[3] with arr[3]
+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |
+---+---+---+---+---+
              i=3 ^
               pivot

Place pivot: swap arr[i+1] with arr[4]
+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |  ← 9 is now in correct position
+---+---+---+---+---+
  ^^^^^^^^^^^^^ ^
   left part  pivot

Pivot index = 4
Left partition: [7,3,5,2]  Right partition: [] (empty)


========================================
PARTITION 2: Left partition [7,3,5,2]
========================================
Range: left=0, right=3
Pivot: Choose last element = 2 (index 3)

+---+---+---+---+---+
| 7 | 3 | 5 | 2 | 9 |  ← pivot = 2
+---+---+---+---+---+
  ^           ^
            pivot

i = -1, j = 0

j=0: arr[0]=7 > 2 → no swap
j=1: arr[1]=3 > 2 → no swap
j=2: arr[2]=5 > 2 → no swap

Place pivot: swap arr[i+1=0] with arr[3]
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |  ← 2 is now in correct position
+---+---+---+---+---+
  ^   ^^^^^^^^^^^^
pivot  right part

Pivot index = 0
Left partition: [] (empty)  Right partition: [3,5,7]


========================================
PARTITION 3: Right partition [3,5,7]
========================================
Range: left=1, right=3
Pivot: Choose last element = 7 (index 3)

+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |  ← pivot = 7
+---+---+---+---+---+
      ^       ^
            pivot

i = 0, j = 1

j=1: arr[1]=3 < 7 → increment i to 1, swap arr[1] with arr[1]
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |
+---+---+---+---+---+
      i=1     ^
            pivot

j=2: arr[2]=5 < 7 → increment i to 2, swap arr[2] with arr[2]
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |
+---+---+---+---+---+
          i=2 ^
            pivot

Place pivot: swap arr[i+1=3] with arr[3]
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |  ← 7 is now in correct position
+---+---+---+---+---+
      ^^^^^^  ^
    left part pivot

Pivot index = 3
Left partition: [3,5]  Right partition: [] (empty)


========================================
PARTITION 4: Left partition [3,5]
========================================
Range: left=1, right=2
Pivot: Choose last element = 5 (index 2)

+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |  ← pivot = 5
+---+---+---+---+---+
      ^   ^
        pivot

i = 0, j = 1

j=1: arr[1]=3 < 5 → increment i to 1, swap arr[1] with arr[1]
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |
+---+---+---+---+---+
      i=1 ^
        pivot

Place pivot: swap arr[i+1=2] with arr[2]
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |  ← 5 is now in correct position
+---+---+---+---+---+
      ^   ^
    left  pivot

Pivot index = 2
Left partition: [3]  Right partition: [] (empty)


========================================
BASE CASES
========================================
Partition [3]: size = 1 → already sorted
All partitions complete!


Finally sorted:
-----------------
+---+---+---+---+---+
| 2 | 3 | 5 | 7 | 9 |
+---+---+---+---+---+


Summary:
--------
Time Complexity:
- Average: O(n log n)
- Worst: O(n²) when pivot is always smallest/largest

Space Complexity: O(log n) for recursion stack

Complexity Analysis:
In Quick Sort, we select a pivot element, partition the array around the pivot, and recursively sort the subarrays.

Recurrence Relation:

Best/Average Case:
Let T(n)T(n) be the time to sort an array of size nn.

  • Selecting a pivot and partitioning the array takes Θ(n)\Theta(n) time.
  • In the best case, the pivot divides the array into two equal halves of size n/2n/2 each.
  • Recursively sorting each half takes T(n/2)T(n/2) time.

Therefore, the recurrence is: T(n)=2T(n/2)+Θ(n)T(n) = 2T(n/2) + \Theta(n)
with base case: T(1)=Θ(1)T(1) = \Theta(1)

Solving using Master Theorem:
The recurrence is of the form: T(n)=aT(n/b)+f(n)T(n) = aT(n/b) + f(n)
where a=2a = 2, b=2b = 2, and f(n)=Θ(n)f(n) = \Theta(n).

We compute: nlog⁡ba=nlog⁡22=n1=nn^{\log_b a} = n^{\log_2 2} = n^1 = n

Since f(n)=Θ(n)=Θ(nlog⁡ba)f(n) = \Theta(n) = \Theta(n^{\log_b a}), we are in Case 2 of the Master Theorem.

Therefore: T(n)=Θ(nlog⁡balog⁡n)=Θ(nlog⁡n)T(n) = \Theta(n^{\log_b a} \log n) = \boxed{\Theta(n \log n)}

Worst Case:
In the worst case, the pivot is always the smallest or largest element, resulting in one subarray of size n−1n-1 and one of size 00.

The recurrence becomes: T(n)=T(n−1)+Θ(n)T(n) = T(n-1) + \Theta(n)
with base case: T(1)=Θ(1)T(1) = \Theta(1)

Expanding the recurrence:
T(n)=T(n−1)+Θ(n)=T(n−2)+Θ(n−1)+Θ(n)=⋯=Θ(1+2+⋯+n)=Θ(n2)T(n) = T(n-1) + \Theta(n) = T(n-2) + \Theta(n-1) + \Theta(n) = \dots = \Theta(1 + 2 + \dots + n) = \boxed{\Theta(n^2)}

For more animations, visit: https://dsa-experiments.vercel.app/recursion/quick-sort

Quicksort Algorithm Visualization

A step-by-step visualization of the quicksort partitioning process

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