LeetCode 24. Swap Nodes in Pairs

24. Swap Nodes in Pairs

My Submissions QuestionEditorial Solution

Total Accepted: 91202 Total Submissions: 260548 Difficulty: Easy

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.

/**
* 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) {
if(head == NULL || head->next == NULL) {
return head;
}
ListNode *p, *q, *h, *t;
p = head;
q = head->next;
h = q;
while(p != NULL && q != NULL) {
p->next = q->next;
q->next = p;
t = p;
if(p->next != NULL) {
p = p->next;
} else {
return h;
}
if(p->next != NULL) {
q = p->next;
t->next = q;
} else {
return h;
}
}
}
};