[LeetCode 024] Swap Nodes in Pairs
Question
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.
Codes
Iterative version
public class Solution {
public ListNode swapPairs(ListNode head) {
if (head == null) {
return null;
}
ListNode anchor = new ListNode(0); // add a new node pointing to head
anchor.next = head;
ListNode pre = anchor; // the node before the pair
ListNode next; // the node which the second node in pair points to
ListNode node1; // the first node in pair
while (pre.next != null && pre.next.next != null) {
node1 = pre.next; // initial the first node
next = pre.next.next.next;
node1.next.next = node1; // node1.next is the second node; set the second node pointing to the first node
pre.next = node1.next; // set the node before the pair pointing to the second node
node1.next = next; // set the first node pointing to next node
pre = pre.next.next; // initial pre node
}
return anchor.next;
}
}
Recursive version
public class Solution {
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode newPair = head.next.next;
ListNode newHead = head.next;
head.next.next = head;
head.next = swapPairs(newPair);
return newHead;
}
}
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