Add solution 24.

Author: javadev

src/Algorithms/0024.swap-nodes-in-pairs/swap-nodes-in-pairs.php

@@ -0,0 +1,27 @@
+/**
+ * Definition for a singly-linked list.
+ * class ListNode {
+ *     public $val = 0;
+ *     public $next = null;
+ *     function __construct($val) { $this->val = $val; }
+ * }
+ */
+class Solution {
+
+    /**
+     * @param ListNode $head
+     * @return ListNode
+     */
+    function swapPairs($head) {
+        $cur = $head;
+        while ($cur != null && $cur->next != null){
+           
+            $temp = $cur->val;
+            $cur->val = $cur->next->val;
+            $cur->next->val = $temp;
+            $cur = $cur->next->next;
+        }
+
+        return $head;
+    }
+}