solves swap nodes in pairs

Author: anishLearnsToCode

README.md

@@ -31,6 +31,7 @@
 |  20  | [ValidParentheses](https://leetcode.com/problems/valid-parentheses/)                                                                                      |                                  [![Java](assets/java.png)](src/ValidParentheses.java) [![Python](assets/python.png)](python/valid_parentheses.py)                                   |                                                                                                                                   |
 |  21  | [Merge 2 Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)                                                                             |                                [![Java](assets/java.png)](src/Merge2SortedLists.java) [![Python](assets/python.png)](python/merge_2_sorted_lists.py)                                 |                                                                                                                                   |
 |  22  | [Generate Parentheses](https://leetcode.com/problems/generate-parentheses)                                                                                |                                                               [![Java](assets/java.png)](src/GenerateParentheses.java)                                                               |                                                                                                                                   |
+|  24  | [Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs)                                                                                  |                                                                [![Java](assets/java.png)](src/SwapNodesInPairs.java)                                                                 |                                                                                                                                   |
 |  26  | [Remove Duplicates From Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)                                                 |                  [![Java](assets/java.png)](src/RemoveDuplicatesFromSortedArray.java) [![Python](assets/python.png)](python/remove_duplicates_from_sorted_array.py)                  |                                                                                                                                   |
 |  27  | [Remove Element](https://leetcode.com/problems/remove-element/)                                                                                           |                                     [![Java](assets/java.png)](src/RemoveElement.java) [![Python](assets/python.png)](python/remove_element.py)                                      |                                                                                                                                   |
 |  28  | [Needle in Haystack](https://leetcode.com/problems/implement-strstr)                                                                                      |                                  [![Java](assets/java.png)](src/NeedleInHaystack.java) [![Python](assets/python.png)](python/needle_in_haystack.py)                                  |                                                                                                                                   |

src/SwapNodesInPairs.java

@@ -0,0 +1,29 @@
+// https://leetcode.com/problems/swap-nodes-in-pairs
+// T: O(n)
+// S: O(1)
+
+public class SwapNodesInPairs {
+    private static class ListNode {
+        int val;
+        ListNode next;
+
+        ListNode(int val, ListNode next) {
+            this.val = val;
+            this.next = next;
+        }
+    }
+
+    public ListNode swapPairs(ListNode head) {
+        if (head == null || head.next == null) return head;
+        ListNode newHead = head.next, a = head, b = head.next.next, temp;
+        head.next.next = head;
+        while (a != null) {
+            a.next = (b == null || b.next == null ? b : b.next);
+            temp = (b == null || b.next == null ? null : b.next.next);
+            if (b != null && b.next != null) b.next.next = b;
+            a = b;
+            b = temp;
+        }
+        return newHead;
+    }
+}