Swap adjucent nodes without modifying node's data

Author: Antesh Sharma

src/main/java/com/antesh/dsa/linkedlist/SwapNodesInPair.java

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+package com.antesh.dsa.linkedlist;
+
+/*
+* 24. Swap Nodes in Pairs
+* Given a linked list, swap every two adjacent nodes and return its head.
+* You must solve the problem without modifying the values in the list's
+* nodes (i.e., only nodes themselves may be changed.)
+*
+* Input: head = [1,2,3,4]
+* Output: [2,1,4,3]
+*
+* Input: head = []
+* Output: []
+*
+* Input: head = [1]
+* Output: [1]
+* */
+
+import java.util.Objects;
+
+public class SwapNodesInPair {
+
+    public static ListNode head;
+
+    class ListNode {
+        int data;
+        ListNode next;
+
+        public ListNode() {
+        }
+
+        public ListNode(int data) {
+            this.data = data;
+        }
+    }
+
+    public ListNode insert(ListNode head, int data) {
+        if (Objects.isNull(head)) {
+            return new ListNode(data);
+        }
+        ListNode temp = new ListNode(data);
+        ListNode ptr = head;
+
+        while (ptr.next != null) {
+            ptr = ptr.next;
+        }
+        ptr.next = temp;
+
+        return head;
+    }
+
+    public ListNode swapPairs(ListNode head) {
+
+        if (head == null || head.next == null) {
+            return head;
+        }
+
+        ListNode current = head;
+        ListNode dummy = new ListNode();
+        dummy.next = head;
+        ListNode previous = dummy;
+
+        while(current != null && current.next != null) {
+            previous.next = current.next;
+            current.next = previous.next.next;
+            previous.next.next = current;
+            previous = current;
+            current = current.next;
+        }
+        return dummy.next;
+    }
+
+    public void print(ListNode head) {
+        ListNode ptr = head;
+        System.out.print("[ ");
+        while (ptr != null) {
+            System.out.print(ptr.data + " --> ");
+            ptr = ptr.next;
+        }
+        System.out.print(" ]");
+        System.out.println();
+    }
+
+    public static void main(String[] args) {
+        SwapNodesInPair swap = new SwapNodesInPair();
+
+        head = swap.insert(head, 1);
+        head = swap.insert(head, 2);
+        head = swap.insert(head, 3);
+        head = swap.insert(head, 4);
+
+        swap.print(head);
+
+        ListNode result = swap.swapPairs(head);
+
+        swap.print(result);
+    }
+}