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.
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class Solution {
public:
ListNode* (ListNode* head) {
if (head == NULL || head->next == NULL)
return head;
ListNode *root = head->next;
head->next = root->next;
root->next = head;
ListNode *p = root->next;
while (p->next != NULL && p->next->next != NULL) {
ListNode *one = p->next;
p->next = one->next;
one->next = p->next->next;
p->next->next = one;
p = p->next->next;
}
return root;
}
};
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