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C/C++ Algorithms

Quick Sort

Partition values around a pivot.

What is Quick Sort?

Quick sort places a pivot in its final position, then recursively sorts the two partitions.

Important points

  • Test empty, single-element, duplicate, and already sorted inputs.
  • Separate correctness from optimization.
  • Use the standard library in production unless a custom implementation is justified.

Quick Sort visualizer

O(n log n)

Use Play or Step to follow each comparison and data movement.

C and C++ code examples

C
Run code →
main.c
#include <stdio.h>

void swap(int *left, int *right) {
    int temporary = *left;
    *left = *right;
    *right = temporary;
}

int partition(int *values, int low, int high) {
    int pivot = values[high];
    int boundary = low;
    for (int scan = low; scan < high; ++scan) {
        if (values[scan] < pivot) {
            swap(&values[boundary++], &values[scan]);
        }
    }
    swap(&values[boundary], &values[high]);
    return boundary;
}

void quick_sort(int *values, int low, int high) {
    if (low >= high) return;
    int pivot = partition(values, low, high);
    quick_sort(values, low, pivot - 1);
    quick_sort(values, pivot + 1, high);
}

int main(void) {
    int values[] = {9, 4, 7, 3, 10, 5};
    quick_sort(values, 0, 5);
    for (int index = 0; index < 6; ++index) {
        printf("%d ", values[index]);
    }
}
Result of running the code
3 4 5 7 9 10
C++
Run code →
main.cpp
#include <algorithm>
#include <iostream>
#include <vector>

template<class Iterator>
void quick_sort(Iterator first, Iterator last) {
    if (last - first < 2) return;
    auto pivot = *(last - 1);
    auto middle = std::partition(first, last - 1,
        [pivot](int value) { return value < pivot; });
    std::iter_swap(middle, last - 1);
    quick_sort(first, middle);
    quick_sort(middle + 1, last);
}

int main() {
    std::vector values{9, 4, 7, 3, 10, 5};
    quick_sort(values.begin(), values.end());
    for (int value : values) {
        std::cout << value << ' ';
    }
}
Result of running the code
3 4 5 7 9 10

C and C++ comparison

Average time is O(n log n), but poor pivots can produce O(n²). The C++ example uses std::partition to express the partition step while retaining the algorithm.

C

Loops, pointers, lengths, and temporary buffers are written explicitly.

C++

Iterators and standard algorithms separate operations from container representation.

Practice exercises

Run both versions, then modify them to observe the different language guarantees.

  • Trace each comparison on paper.
  • Test duplicates and extreme values.
  • Benchmark the custom implementation against the standard library.

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