Thread: What is problem about the sorting?

The merge sort is written by myself

Code:

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

void merge(int *a, int p, int q, int r)
{
	int n1 = q-p+1;
	int n2 = r-q;
	int *L = (int*)malloc((n1+1)*sizeof(int));
	int *R = (int*)malloc((n2+1)*sizeof(int));
	int i, j, k;
	for (i = 0; i < n1; i++)
		L[i] = a[p+i];
	for (j = 0; j < n2; j++)
		R[j] = a[q+j];
	L[n1+1] = -1;
	R[n2+1] = -1;
	i = 0;
	j = 0;
	for (k = p; k < r; k++)
		if (L[i] <= R[j]) {
			a[k] = L[i];
			i++;
		}
		else {
			a[k] = R[j];
			j++;
		}
	free(L);
	free(R);
}

void merge_sort(int *a, int p, int r)
{
	if (p < r) {
		int q = (int)((p+r)/2);
		merge_sort(a, p, q);
		printf("asdfasd\n");
		merge_sort(a, q+1, r);
		printf("haha\n");
		merge(a, p, q, r);
	}
}

int main()
{
	int i, n;
	int *a;
	printf("n = ");
	scanf("%d", &n);
	for (i = 0; i < n; i++)
		scanf("%d", &a[i]);
	merge_sort(a, 0, n-1);
	for (i = 0; i < n; i++)
		printf("%d ", a[i]);
	return 0;
}

Why doesn't this code work fine? Which step do i make wrong?