24. 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.

解法一：遞迴

public:
    ListNode* swapPairs(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode* new_head = head->next;
        head->next = swapPairs(new_head->next);
        new_head->next = head;
        return new_head;
    }
};

解法二：非遞迴

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
       ListNode* new_head = NULL;
       ListNode** pp = &new_head;
       while(head != NULL){
           if(head->next == NULL){
               *pp = head;
               break;
           }
           else{
               *pp = head->next;
               head->next = (*pp)->next;
               (*pp)->next = head;
               pp = &(head->next);
               head = head->next;
           }
       }
       return new_head;
    }
};