diff --git a/alien-dictionary/PDKhan.cpp b/alien-dictionary/PDKhan.cpp
new file mode 100644
index 000000000..9ca24c575
--- /dev/null
+++ b/alien-dictionary/PDKhan.cpp
@@ -0,0 +1,67 @@
+class Solution {
+public:
+    string alienOrder(vector<string> &words) {
+        // Write your code here
+        unordered_map<char, unordered_set<char>> graph;
+        unordered_map<char, int> indegree;
+        unordered_set<char> all_chars;
+
+        for (const string& word : words) {
+            for (char c : word) {
+                all_chars.insert(c);
+                if (!indegree.count(c))
+                    indegree[c] = 0;
+            }
+        }
+
+        for(int i = 0; i < words.size() - 1; i++){
+            string word1 = words[i];
+            string word2 = words[i + 1];
+            int len = min(word1.size(), word2.size());
+            bool found = false;
+
+            for(int j = 0; j < len; j++){
+                char ch1 = word1[j];
+                char ch2 = word2[j];
+
+                if(ch1 != ch2){
+                    if(graph[ch1].count(ch2) == 0){
+                        graph[ch1].insert(ch2);
+                        indegree[ch2]++;
+                    }
+
+                    found = true;
+                    break;
+                }
+            }
+
+            if(!found && word1.size() > word2.size())
+                return "";
+        }
+
+        queue<char> q;
+        for(char c : all_chars){
+            if(indegree[c] == 0)
+                q.push(c);
+        }
+
+        string order;
+
+        while(!q.empty()){
+            char cur = q.front();
+            q.pop();
+
+            order += cur;
+
+            for(char next : graph[cur]){
+                if(--indegree[next] == 0)
+                    q.push(next);
+            }
+        }
+
+        if(order.size() != all_chars.size())
+            return "";
+
+        return order;
+    }
+};
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/PDKhan.cpp b/construct-binary-tree-from-preorder-and-inorder-traversal/PDKhan.cpp
new file mode 100644
index 000000000..194a76fcd
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/PDKhan.cpp
@@ -0,0 +1,29 @@
+class Solution {
+public:
+    TreeNode* build(vector<int>& preorder, int& pos, vector<int>& inorder, int start, int end) {
+        if(start >= end)
+            return nullptr;
+
+        int i;
+
+        for(i = start; i < end; i++){
+            if(preorder[pos] == inorder[i])
+                break;
+        }
+
+        TreeNode* root = new TreeNode(preorder[pos]);
+
+        pos++;
+
+        root->left = build(preorder, pos, inorder, start, i);
+        root->right = build(preorder, pos, inorder, i + 1, end);
+
+        return root;
+    }
+
+    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
+        int pos = 0;
+
+        return build(preorder, pos, inorder, 0, inorder.size());
+    }
+};
diff --git a/longest-palindromic-substring/PDKhan.cpp b/longest-palindromic-substring/PDKhan.cpp
new file mode 100644
index 000000000..8e44e39ae
--- /dev/null
+++ b/longest-palindromic-substring/PDKhan.cpp
@@ -0,0 +1,26 @@
+class Solution {
+public:
+    void compare(string s, int left, int right, int& max_start, int& max_end){
+        while(left >= 0 && right < s.length() && s[left] == s[right]){
+            left--;
+            right++;
+        }
+
+        if(max_end - max_start < right - left - 1){
+            max_start = left + 1;
+            max_end = right - 1;
+        }
+    }
+
+    string longestPalindrome(string s) {
+        int max_start = 0;
+        int max_end = 0;
+
+        for(int i = 0; i < s.length(); i++){
+            compare(s, i, i, max_start, max_end);
+            compare(s, i, i + 1, max_start, max_end);
+        }
+
+        return s.substr(max_start, max_end - max_start + 1);
+    }
+};
diff --git a/rotate-image/PDKhan.cpp b/rotate-image/PDKhan.cpp
new file mode 100644
index 000000000..6766df6ff
--- /dev/null
+++ b/rotate-image/PDKhan.cpp
@@ -0,0 +1,15 @@
+class Solution {
+public:
+    void rotate(vector<vector<int>>& matrix) {
+        int n = matrix.size();
+
+        for(int i = 0; i < n; i++){
+            for(int j = i + 1; j < n; j++){
+                swap(matrix[i][j], matrix[j][i]);
+            }
+        }
+
+        for(int i = 0; i < n; i++)
+            reverse(matrix[i].begin(), matrix[i].end());
+    }
+};
diff --git a/subtree-of-another-tree/PDKhan.cpp b/subtree-of-another-tree/PDKhan.cpp
new file mode 100644
index 000000000..19b3c7444
--- /dev/null
+++ b/subtree-of-another-tree/PDKhan.cpp
@@ -0,0 +1,21 @@
+class Solution {
+public:
+    bool compare_tree(TreeNode* root, TreeNode* subRoot) {
+        if(!root && !subRoot)
+            return true;
+        else if(!root || !subRoot || root->val != subRoot->val)
+            return false;
+        
+        return compare_tree(root->left, subRoot->left) && compare_tree(root->right, subRoot->right);
+    }
+
+    bool isSubtree(TreeNode* root, TreeNode* subRoot) {
+        if(!root)
+            return false;
+        
+        if(compare_tree(root, subRoot))
+            return true;
+
+        return isSubtree(root->left, subRoot) || isSubtree(root->right, subRoot);
+    }
+};