C++ Pointer to Pointer

pairSwap.cpp

// the solution by StefanPochmann https://leetcode.com/problems/swap-nodes-in-pairs/discuss/11019/7-8-lines-C++-Python-Ruby 
ListNode* swapPairs(ListNode* head) {
    ListNode **pp = &head, *a, *b;
    while ((a = *pp) && (b = a->next)) {
        a->next = b->next;
        b->next = a;
        *pp = b;
        pp = &(a->next);
    }
    return head;
}


// Linus on Understanding Pointers: https://grisha.org/blog/2013/04/02/linus-on-understanding-pointers/

typedef struct list_entry {
    int val;
    struct list_entry *next;
} list_entry;

list_entry *entry = head; /* assuming head exists and is the first entry of the list */
list_entry *prev = NULL;

//in this case have to consider whether prev is null
while (entry) {
    if (entry->val == to_remove)     /* this is the one to remove */
        if (prev)
           prev->next = entry->next; /* remove the entry */
        else
            head = entry->next;      /* special case - first entry */

    /* move on to the next entry */
    prev = entry;
    entry = entry->next;
}

// now use pointer to pointer don't have to consider first node is null pointer
list_entry **pp = &head; /* pointer to a pointer */
list_entry *entry = head;

while (entry) {
    if (entry->val == to_remove)
        *pp = entry->next;

    pp = &entry->next;
    entry = entry->next;
}