[LeetCode] Swap Nodes in Pairs

Well, since the head pointer may also been modified, we create a new_head that points to it to facilitate the swapping process.

For the example list 1 -> 2 -> 3 -> 4 in the problem statement, it will become 0 -> 1 -> 2 -> 3 -> 4 (we init new_head -> val to be 0). Then we set a pointer pre to new_head and anothercur to head. Each time, we will swap pre -> next and cur -> next using the following piece of code.

pre -> next = cur -> next;
cur -> next = cur -> next -> next;
pre -> next -> next = cur;

After swapping them, we update as follows:

pre = cur; 
cur = pre -> next; 

to swap the next two nodes.

Finally, we return new_head -> next.

The complete code is as follows.

class Solution {
public:
    ListNode* swapPairs(ListNode* head) { 
        if (!head || !(head -> next)) return head;
        ListNode* new_head = new ListNode(0);
        new_head -> next = head;
        ListNode* pre = new_head; 
        ListNode* cur = head;
        while (pre -> next && cur -> next) {
            pre -> next = cur -> next;
            cur -> next = cur -> next -> next;
            pre -> next -> next = cur;
            pre = cur;
            cur = pre -> next;
        }
        return new_head -> next;
    }
};