Merge pull request SnowScriptWinterOfCode#527 from aishwarya-chandra/…

…day22q3

day22q3

Day-22/q3: Sort List/aishwarya--J.md

@@ -0,0 +1,51 @@
+```
+public class Solution {
+  
+  public ListNode sortList(ListNode head) {
+    if (head == null || head.next == null)
+      return head;
+        
+    // step 1. cut the list to two halves
+    ListNode prev = null, slow = head, fast = head;
+    
+    while (fast != null && fast.next != null) {
+      prev = slow;
+      slow = slow.next;
+      fast = fast.next.next;
+    }
+    
+    prev.next = null;
+    
+    // step 2. sort each half
+    ListNode l1 = sortList(head);
+    ListNode l2 = sortList(slow);
+    
+    // step 3. merge l1 and l2
+    return merge(l1, l2);
+  }
+  
+  ListNode merge(ListNode l1, ListNode l2) {
+    ListNode l = new ListNode(0), p = l;
+    
+    while (l1 != null && l2 != null) {
+      if (l1.val < l2.val) {
+        p.next = l1;
+        l1 = l1.next;
+      } else {
+        p.next = l2;
+        l2 = l2.next;
+      }
+      p = p.next;
+    }
+    
+    if (l1 != null)
+      p.next = l1;
+    
+    if (l2 != null)
+      p.next = l2;
+    
+    return l.next;
+  }
+
+}
+```