swap nodes in pairs.cpp

class Solution {
public:
    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;
    }    
};