24. Swap Nodes in Pairs

2019-11-08 01:02:43

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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：递归

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：非递归

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; }};

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