[leetcode] Swap Nodes in Pairs

Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

 

class Solution
{
public:
  ListNode *swapPairs(ListNode *head)
  {
    if(head == NULL) return NULL;
    ListNode *cur = head->next, *pre = head, *temp = NULL;
    int i = 0;
    while(cur)
    {
      if(i % 2 == 0)
      {
        pre->next = cur->next;
        cur->next = pre;
        if(i == 0)
        {
          head = cur;
          temp = head;
        }
        else
        {
          temp->next = cur;
          temp = temp->next;
        }
        cur = pre->next;
      }
      else
      {
        temp = temp->next;
        pre = pre->next;
        cur = cur->next;
      }
      i++;
    }
    return head;
  }
};