k-way merge on 2D array of sorted arrays using bottom up mergesort

kway-mergesort.c

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

void merge(int **a, int start, int end, int sz) {
    int *a1 = a[start];
    int *a2 = a[end];
    int res[sz];
    int i = 0;
    int j = 0;
    int idx = 0;
    while (i < sz / 2 && j < sz / 2) {
        if (a1[i] < a2[j]) {
            res[idx++] = a1[i++];
        } else {
            res[idx++] = a2[j++];
        }
    }
    
    while (i < sz / 2) {
        res[idx++] = a1[i++];
    }
    
    while (j < sz / 2) {
        res[idx++] = a2[j++];
    }
    idx--;
    while (idx >= 0) {
        a[start][idx] = res[idx];
        idx--;
    }
}

int main(int argc, const char * argv[])
{
    int k = 4;
    int n = 4;
    int **a = (int**) malloc(sizeof (int*) * k);
    for (int i = 0; i < k; i++) {
        a[i] = (int*) malloc (sizeof (int) * n);
        memset(a[i], 0, sizeof(int));
    }
    a[0][0] = 1;
    a[0][1] = 24;
    a[0][2] = 47;
    a[0][3] = 54;
    a[1][0] = 5;
    a[1][1] = 6;
    a[1][2] = 12;
    a[1][3] = 14;
    a[2][0] = 9;
    a[2][1] = 10;
    a[2][2] = 11;
    a[2][3] = 18;
    a[3][0] = 13;
    a[3][1] = 16;
    a[3][2] = 19;
    a[3][3] = 23;
    for (int i = 1; i < k; i *= 2) {
        for (int lo = 0; lo < n - i; lo += i * 2) {
            merge(a, lo, lo + i, n * i * 2);
        }
    }
    for (int i = 0; i < k * n; i++) {
        printf("%d", a[0][i]);
        if (i < (k * n - 1)) {
            printf(", ");
        }
    }
    printf("\n");
    free(a);
    return 0;
}

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