merge_sort.c

#include <stdio.h>
#include <stdlib.h>

void merge(int arr[], int p, int q, int r) {
    // Create L ← A[p..q] and M ← A[q+1..r]
    int n1 = q - p + 1;
    int n2 = r - q;

    int L[n1], M[n2];

    for (int i = 0; i < n1; i++) L[i] = arr[p + i];
    for (int j = 0; j < n2; j++) M[j] = arr[q + 1 + j];

    // Maintain current index of sub-arrays and main array
    int i, j, k;
    i = 0;
    j = 0;
    k = p;

    // Until we reach either end of either L or M, pick larger among
    // elements L and M and place them in the correct position at A[p..r]
    while (i < n1 && j < n2) {
        if (L[i] <= M[j]) {
            arr[k] = L[i];
            i++;
        } else {
            arr[k] = M[j];
            j++;
        }
        k++;
    }

    // When we run out of elements in either L or M,
    // pick up the remaining elements and put in A[p..r]
    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }

    while (j < n2) {
        arr[k] = M[j];
        j++;
        k++;
    }
}

void merge_sort(int arr[], int l, int r) {
    if (l < r) {
        // m is the point where the array is divided into two subarrays
        int m = l + (r - l) / 2;

        merge_sort(arr, l, m);
        merge_sort(arr, m + 1, r);

        // Merge the sorted subarrays
        merge(arr, l, m, r);
    }
}

int main(int argc, char** argv) {
    int arr[] = { 5, 4, 3, 2, 1 };
    int n = sizeof(arr) / sizeof(arr[0]);

    merge_sort(arr, 0, n - 1);

    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }

    return 0;
}