diff --git a/binary-tree-maximum-path-sum/PDKhan.cpp b/binary-tree-maximum-path-sum/PDKhan.cpp
new file mode 100644
index 000000000..8a4bf0fb8
--- /dev/null
+++ b/binary-tree-maximum-path-sum/PDKhan.cpp
@@ -0,0 +1,22 @@
+class Solution {
+    public:
+        int dfs(TreeNode* root, int& result){
+            if(root == nullptr)
+                return 0;
+            
+            int left = max(0, dfs(root->left, result));
+            int right = max(0, dfs(root->right, result));
+            int sum = root->val + left + right;
+    
+            result = max(result, sum);
+    
+            return root->val + max(left, right);
+        }
+    
+        int maxPathSum(TreeNode* root) {
+            int result = INT_MIN;
+    
+            dfs(root, result);
+            return result;
+        }
+    };
diff --git a/graph-valid-tree/PDKhan.cpp b/graph-valid-tree/PDKhan.cpp
new file mode 100644
index 000000000..a1588f24b
--- /dev/null
+++ b/graph-valid-tree/PDKhan.cpp
@@ -0,0 +1,46 @@
+class Solution {
+    public:
+        bool dfs(int curr, int parent, vector<vector<int>>& graph, vector<int>& visited){
+            if(visited[curr])
+                return false;
+    
+            visited[curr] = 1;
+    
+            for(int i = 0; i < graph[curr].size(); i++){
+                if(graph[curr][i] == parent)
+                    continue;
+    
+                if(dfs(graph[curr][i], curr, graph, visited) == false)
+                    return false;
+            }
+    
+            return true;
+        }
+    
+        bool validTree(int n, vector<vector<int>> &edges) {
+            // write your code here
+            if(edges.size() != n - 1)
+                return false;
+    
+            vector<vector<int>> graph (n);
+            vector<int> visited (n, 0);
+    
+            for(int i = 0; i < edges.size(); i++){
+                int first = edges[i][0];
+                int second = edges[i][1];
+    
+                graph[first].push_back(second);
+                graph[second].push_back(first);
+            }
+    
+            if(dfs(0, -1, graph, visited) == false)
+                return false;
+            
+            for(int i = 0; i < n; i++){
+                if(visited[i] == 0)
+                    return false;
+            }
+    
+            return true;
+        }
+    };
diff --git a/merge-intervals/PDKhan.cpp b/merge-intervals/PDKhan.cpp
new file mode 100644
index 000000000..1dce46c12
--- /dev/null
+++ b/merge-intervals/PDKhan.cpp
@@ -0,0 +1,23 @@
+class Solution {
+    public:
+        vector<vector<int>> merge(vector<vector<int>>& intervals) {
+            if(intervals.empty())
+                return {};
+            
+            sort(intervals.begin(), intervals.end(), [](const vector<int>& a, const vector<int>& b){
+                return a[0] < b[0];
+            });
+    
+            vector<vector<int>> result;
+            result.push_back(intervals[0]);
+    
+            for(int i = 0; i < intervals.size(); i++){
+                if(result.back()[1] >= intervals[i][0])
+                    result.back()[1] = max(result.back()[1], intervals[i][1]);
+                else
+                    result.push_back(intervals[i]);
+            }
+    
+            return result;
+        }
+    };
diff --git a/missing-number/PDKhan.cpp b/missing-number/PDKhan.cpp
new file mode 100644
index 000000000..cae451a1e
--- /dev/null
+++ b/missing-number/PDKhan.cpp
@@ -0,0 +1,14 @@
+class Solution {
+    public:
+        int missingNumber(vector<int>& nums) {
+            int sum1 = 0;
+            int sum2 = 0;
+    
+            for(int i = 0; i < nums.size(); i++){
+                sum1 += nums[i];
+                sum2 += i + 1;
+            }
+    
+            return sum2 - sum1;
+        }
+    };
diff --git a/reorder-list/PDKhan.cpp b/reorder-list/PDKhan.cpp
new file mode 100644
index 000000000..ff6a6052b
--- /dev/null
+++ b/reorder-list/PDKhan.cpp
@@ -0,0 +1,73 @@
+// space O(n)
+class Solution {
+    public:
+        void reorderList(ListNode* head) {
+            ListNode* search = head->next;
+            ListNode* tail = head;
+            deque<ListNode*> q;
+    
+            while(search){
+                ListNode* next = search->next;
+    
+                search->next = NULL;
+                q.push_back(search);
+                search = next;
+            }
+    
+            for(int i = 0; !q.empty(); i++){
+                if(i % 2 == 0){
+                    tail->next = q.back();
+                    q.pop_back();
+                }else{
+                    tail->next = q.front();
+                    q.pop_front();
+                }
+    
+                tail = tail->next;
+            }
+        }
+    };
+
+
+// space O(1)
+class Solution {
+    public:
+        void reorderList(ListNode* head) {
+            if(head == NULL || head->next == NULL)
+                return;
+    
+            ListNode* slow = head;
+            ListNode* fast = head;
+    
+            while(fast->next && fast->next->next){
+                slow = slow->next;
+                fast = fast->next->next;
+            }
+    
+            ListNode* prev = nullptr;
+            ListNode* curr = slow->next;
+    
+            while(curr){
+                ListNode* next = curr->next;
+                curr->next = prev;
+                prev = curr;
+                curr = next;
+            }
+    
+            slow->next = nullptr;
+    
+            ListNode* first = head;
+            ListNode* second = prev;
+    
+            while(first && second){
+                ListNode* next1 = first->next;
+                ListNode* next2 = second->next;
+    
+                first->next = second;
+                second->next = next1;
+    
+                first = next1;
+                second = next2;
+            }
+        }
+    };