static void *extend_heap(size_t words)
{
    char *bp;
    size_t size;
        
    /* Allocate an even number of words to maintain alignment */
    size = (words % 2) ? (words+1) * WSIZE : words * WSIZE;
    if ((int)(bp = mem_sbrk(size)) < 0)
        return NULL;

    /* Initialize free block header/footer and the epilogue header */
    PUT(HDRP(bp), PACK(size, 0));         /* free block header */
    PUT(FTRP(bp), PACK(size, 0));         /* free block footer */
    PUT(HDRP(NEXT_BLKP(bp)), PACK(0, 1)); /* new epilogue header */

    /* Coalesce if the previous block was free */
    return coalesce(bp);
}