diff --git a/merge-intervals/ppxyn1.py b/merge-intervals/ppxyn1.py
new file mode 100644
index 0000000000..5d169b48f6
--- /dev/null
+++ b/merge-intervals/ppxyn1.py
@@ -0,0 +1,17 @@
+# idea : -
+# TimeComplexity: # O(n log(n))
+class Solution:
+    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
+        intervals = sorted(intervals) 
+        output = [intervals[0]]
+
+        # Merge when intervals overlap
+        for start, end in intervals[1:]:
+            lastEnd = output[-1][1] 
+            if start <= lastEnd:
+                output[-1][1] = max(lastEnd, end)
+            else:
+                output.append([start, end])
+        return output
+
+
diff --git a/missing-number/ppxyn1.py b/missing-number/ppxyn1.py
new file mode 100644
index 0000000000..7675528a70
--- /dev/null
+++ b/missing-number/ppxyn1.py
@@ -0,0 +1,12 @@
+# idea : - 
+# Time Complexity : below O(n^2)
+
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        nums.sort()      
+        for i in range(len(nums)):
+            if nums[i] != i:
+                return i
+        return len(nums)
+    
+
diff --git a/reorder-list/ppxyn1.py b/reorder-list/ppxyn1.py
new file mode 100644
index 0000000000..3fc6702244
--- /dev/null
+++ b/reorder-list/ppxyn1.py
@@ -0,0 +1,43 @@
+# idea: Two-Pointer 
+# Time Complexixty: O(n)?
+'''
+Linked lists do not provide a len func, split at the middle and merge the two halves sequentially
+'''
+
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reorderList(self, head):
+        if not head or not head.next:
+            return
+
+        # 1. middle
+        slow, fast = head, head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        # 2. reverse second half
+        prev = None
+        cur = slow.next
+        slow.next = None
+        
+        while cur:
+            nxt = cur.next
+            cur.next = prev
+            prev = cur
+            cur = nxt
+
+        # 3. merge
+        first, second = head, prev
+        while second:
+            t1, t2 = first.next, second.next
+            first.next = second
+            second.next = t1
+            first = t1
+            second = t2
+
+