Swap Nodes Pairs in Link List - Leet Code Solution

Problem Statement

Given a linked list, swap every two adjacent nodes and return its head. You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Solution

Lets look at the first two nodes first.
We need to move head to second. (One time operation)
Move next of First to next of Second
Move next of Second to First
All looks fine. We can go like this in pairs.
Now, what about joining the pairs?
We need another pointer who will be responsible of joining these pairs.

Code

public static class ListNode {
	int val;
	ListNode next;
	ListNode(int x) { val = x; }
	@Override
	public String toString() {
		StringBuffer sb = new StringBuffer();
		ListNode t = this;
		while (t != null) {
			sb.append(t.val).append(" -> ");
			t = t.next;
		}
		return sb.toString();
	}
}

public ListNode swapPairs(ListNode head) {
	if (head == null || head.next == null) return head;
	
	ListNode first = head;
	ListNode second = head.next;
	ListNode prev = null;
	
	ListNode result = second;
	while (first != null && second != null) {
		first.next = second.next;
		second.next = first;
		
		if (prev != null) {
			prev.next = second;
		}
		
		prev = first;
		
		first = first.next;
		if (first != null) {
			second = first.next;
		}
	}
	
	return result;
}

Leet Code submission result

Runtime: 0 ms, faster than 100.00% of Java online submissions for Swap Nodes in Pairs. Memory Usage: 34.5 MB, less than 100.00% of Java online submissions for Swap Nodes in Pairs.