LeetCode-24-Swap Nodes in Pairs

算法描述：

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

Example:

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

Note:

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

解题思路：

数据结构题，采用一快一慢两个指针，画出图就能完成，注意的是对快指针的判断。

    ListNode* swapPairs(ListNode* head) {
        ListNode* dup = new ListNode(-1);
        dup->next = head;
        ListNode* cur = dup;
        ListNode* fast = dup;
        ListNode* slow = dup;
        while(fast-> next !=nullptr && fast->next->next!=nullptr){
            fast = fast->next->next;
            slow = slow->next;
            ListNode* temp = fast->next;
            cur->next = fast;
            fast->next = slow;
            slow->next = temp;
            fast = slow;
            cur=slow;
        }
        return dup->next;
    }