[LeetCode] Swap Nodes in Pairs

时间：2015-07-06 15:49:25 收藏：0 阅读：109

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.

 1 class Solution {
 2 public:
 3     ListNode* swapPairs(ListNode* head) { 
 4         if (!head || !(head -> next)) return head;
 5         ListNode* new_head = new ListNode(0);
 6         new_head -> next = head;
 7         ListNode* pre = new_head; 
 8         ListNode* cur = head;
 9         while (pre -> next && cur -> next) {
10             pre -> next = cur -> next;
11             cur -> next = cur -> next -> next;
12             pre -> next -> next = cur;
13             pre = cur;
14             cur = pre -> next;
15         }
16         return new_head -> next;
17     }
18 };