Weird behaviour when merge sorting an array

Question

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/time.h>
#include <time.h>

void DISPLAY(int*, int);

int* MERGE(int*, int,  int* , int,  int* , int ); 
int* MERGESORT(int* , int ); 



int main()
{

    int* array = NULL;
    array = (int*) malloc(7 * sizeof(int) );
    
    array[0] = 10;
    array[1] = 9;
    array[2] = 8;
    array[3] = 57;
    array[4] = 6;
    array[5] = 5;
    array[6] = 24;
    
    printf("\nOriginal array: \n");
    DISPLAY(array, 7);
                
    array = MERGESORT(array, 7);
                
    printf("\n\nResulting array: \n");
    DISPLAY(array, 7);
}

void DISPLAY(int* array, int size)
{
    int i;                                          
    for(i = 0; i < size; i++){                  
        printf("%d ", array[i]);
    }
}


int* MERGESORT(int *array, int size) {
    if(size < 2) 
        return array; 

    int mid = size / 2;             
    int rightsize = size-mid;   

    int* L = (int*) malloc(mid * sizeof(int));         
    int* R = (int*) malloc((size-mid) * sizeof(int));  
    int i;
    for(i = 0; i < mid; i++) 
        L[i] = array[i];                            
    for(i = mid; i < size; i++) 
        R[i-mid] = array[i];                    

    L = MERGESORT(L, mid);                           
    R = MERGESORT(R, size-mid);                     
    array = MERGE(array, size, L, mid, R, rightsize);       
    free(L);                                        
    free(R);                                    
    
    return array;
}


int* MERGE(int *array, int size, int *L, int leftsize,int *R, int rightsize) {
    int i, j, k;
    
    i = 0; 
    j = 0; 
    k = 0;

    while(i < leftsize && j < rightsize)    
    {                   
        if(L[i] < R[j])                     
        {
            array[k] = L[i];                
            i++;                            
        }
        else{                           
            array[k] = R[j];            
            j++;                        
        }
        k++;                        
    }
    while(i < leftsize)                 
    {
        array[k] = L[i];                    
        i++;                                
        k++;                                    
    }
    while(j < rightsize)
    {
        array[k] = L[j];                
        j++;                                
        k++;                            
    }
    
    printf("\nUpdated array: \n");                       
    DISPLAY(array, size);
    return array;
}

Hello, I have a problem when I try to perform merge sorting in an array. Some items in the array are printing weird. Every time the arrays get updated, the value gets printed out, but it doesn't work at specific inputs.

When I input small numbers (like 1-2 digits), then the array gets sorted normally.

Sample run of program sorting incorrectly: https://i.sstatic.net/OLg0b.png

Answer 1

while(j < rightsize)
    {
        array[k] = L[j];                
        j++;                                
        k++;                            
    }

should be

while(j < rightsize)
        {
            array[k] = R[j];                
            j++;                                
            k++;                            
        }

Umm, sorry for wasting time on such a simple mistake.

Answer 2

Consider.

int* R = (int*) malloc((size-mid) * sizeof(int)); 

(which ought to be written int *R = malloc((size - mid) * sizeof *R);)

What is the highest valid index in that array? Trying to read from (or write to) R[n] for n > size - mid - 1 invokes undefined behavior.