From 10410e5ff9a54202b765c46f514accf5fdc41cf7 Mon Sep 17 00:00:00 2001
From: doh6077 <kimdohee@sheridancollege.ca>
Date: Sun, 18 Jan 2026 12:55:32 -0500
Subject: [PATCH 1/5] Missing Number Solution

---
 missing-number/doh6077.py | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
 create mode 100644 missing-number/doh6077.py

diff --git a/missing-number/doh6077.py b/missing-number/doh6077.py
new file mode 100644
index 0000000000..6b2aeb3cb1
--- /dev/null
+++ b/missing-number/doh6077.py
@@ -0,0 +1,16 @@
+# Missing Number
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        # First Solution: Time Complexity: O(n^2)
+        n = len(nums)
+
+        for i in range(0,n + 1):
+            if i not in nums:
+                return i    
+            
+        # Time Complexity: O(n) 
+        n = len(nums)
+        # calculate the sum of first n numbers
+        sum_val = n * (n + 1) // 2
+        return sum_val - sum(nums)
+

From 0f534261d0d2961f34f861e561269aa29e587118 Mon Sep 17 00:00:00 2001
From: doh6077 <kimdohee@sheridancollege.ca>
Date: Sun, 18 Jan 2026 13:47:22 -0500
Subject: [PATCH 2/5] 143. Reorder List Solution

---
 reorder-list/doh6077.py | 43 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 reorder-list/doh6077.py

diff --git a/reorder-list/doh6077.py b/reorder-list/doh6077.py
new file mode 100644
index 0000000000..6c8519fefb
--- /dev/null
+++ b/reorder-list/doh6077.py
@@ -0,0 +1,43 @@
+# 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: Optional[ListNode]) -> None:
+        """
+        Do not return anything, modify head in-place instead.
+        """
+        # Time Complexity: O(N)
+        # The order: 
+        # 0, n, 1, n -1, 2, n-3 ...
+        # first Idea
+        # need to save the index of the original head 
+        # Hashmap: iterate through the head until head it none, save index as key and head.val as value 
+        head_hash = {}
+        temp = head
+        index = 0 
+        length = 0 
+        # Save index and value in the hashmap
+        while temp is not None:
+            head_hash[index] = temp.val
+            temp = temp.next 
+            index += 1
+            length += 1
+        # reset index to 0, and use it to iterate through the head again
+        index = 0 
+        # to keep track of n-1, n-2, n-3 ...
+        count = 1
+        # Iterate through the head again and change the value based on the index
+        while head is not None: 
+            res = index % 2 
+            # if the index is even number 
+            if res == 0:
+                head.val = head_hash[index/2]
+            # n, n-1, n-2 when the index is odd 
+            else:
+                head.val = head_hash[length - count]
+                count += 1 
+            index += 1
+            head = head.next
+            
\ No newline at end of file

From a07710ca3f88dba0616387690fb2969ae665ce87 Mon Sep 17 00:00:00 2001
From: doh6077 <kimdohee@sheridancollege.ca>
Date: Sun, 18 Jan 2026 13:48:33 -0500
Subject: [PATCH 3/5] =?UTF-8?q?=EC=A4=84=EB=B0=94=EA=BF=88?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 reorder-list/doh6077.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/reorder-list/doh6077.py b/reorder-list/doh6077.py
index 6c8519fefb..70d9417451 100644
--- a/reorder-list/doh6077.py
+++ b/reorder-list/doh6077.py
@@ -40,4 +40,3 @@ def reorderList(self, head: Optional[ListNode]) -> None:
                 count += 1 
             index += 1
             head = head.next
-            
\ No newline at end of file

From 144ee0d6018e35b6136e26f0e685d65e4a3c1bf8 Mon Sep 17 00:00:00 2001
From: doh6077 <kimdohee@sheridancollege.ca>
Date: Mon, 19 Jan 2026 12:38:37 -0500
Subject: [PATCH 4/5] graph Valid Tree solution

---
 graph-valid-tree/doh6077.py | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 graph-valid-tree/doh6077.py

diff --git a/graph-valid-tree/doh6077.py b/graph-valid-tree/doh6077.py
new file mode 100644
index 0000000000..97076f7f51
--- /dev/null
+++ b/graph-valid-tree/doh6077.py
@@ -0,0 +1,33 @@
+class Solution:
+    def validTree(self, n: int, edges: List[List[int]]) -> bool:
+        # Valid Tree
+        # 1. no loop
+        # 2. no disconnected node 
+        # Use DFS and Hashset to track visited nodes 
+        if not n:
+            return True
+        
+        adj = {i:[] for i in range(n)}
+        for n1, n2 in edges:
+            adj[n1].append(n2)
+            adj[n2].append(n1)
+        visit = set()
+        def dfs(i, prev):
+            # Base case
+            if i in visit:
+                return False
+            visit.add(i)
+            # check neighbor nodes 
+            for j in adj[i]:
+                if j == prev:
+                    continue
+                if not dfs(j,i):
+                    return False
+            return True
+
+        return dfs(0, -1) and n == len(visit)
+
+
+
+        
+

From 617abb3dce95fdb6488e31b522e5fb4cb20d6994 Mon Sep 17 00:00:00 2001
From: doh6077 <kimdohee@sheridancollege.ca>
Date: Mon, 19 Jan 2026 12:39:04 -0500
Subject: [PATCH 5/5] =?UTF-8?q?=EC=A4=84=EB=B0=94=EA=BF=88?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 graph-valid-tree/doh6077.py | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/graph-valid-tree/doh6077.py b/graph-valid-tree/doh6077.py
index 97076f7f51..7739a13b43 100644
--- a/graph-valid-tree/doh6077.py
+++ b/graph-valid-tree/doh6077.py
@@ -26,8 +26,3 @@ def dfs(i, prev):
             return True
 
         return dfs(0, -1) and n == len(visit)
-
-
-
-        
-