Time Cost

14min52s

Implementation

Quick-Slow Pointers. When the quick pointer hits the boundary then stops.

Code

• Solution

  class Solution {
  public:
    ListNode* swapPairs(ListNode* head) {
        if (head == nullptr || head->next == nullptr) return head;
        // use two-pointers
        ListNode dummy(0);
        dummy.next = head;
  
        ListNode* fast = head->next;
        ListNode* slow = head;
        ListNode* prev = &dummy;
  
        while (fast) {
            ListNode* nxt = fast->next;
            prev->next = fast;
            fast->next = slow;
            slow->next = nxt;
            prev = slow;
            if (!nxt || !nxt->next) {
                break;
            }
            slow = nxt;
            fast = nxt->next;
        }
  
        return dummy.next;
    }
  };