24.Swap Nodes in Pairs

題目描述：

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.

代碼：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        int index = 1;
        ListNode* current = head;
        ListNode* first;
        while(current != NULL){
            if(index % 2 != 0){
                first = current;
                current = current -> next;
            } 
            else{
                int fValue = first -> val;
                first -> val = current -> val;
                current -> val = fValue;
                current = current -> next;
            }
            index++;
        }
        return head;
    }
};

此題目歸爲Medium有點過分感覺，稍微有點鏈表知識的人都知道是很水的一道題，兩兩互換值，只需要每次保存一下奇數位（起始位置爲1）節點然後在偶數位互換val即可。