diff --git a/combination-sum/nhistory.js b/combination-sum/nhistory.js
new file mode 100644
index 000000000..2fd9dd1b5
--- /dev/null
+++ b/combination-sum/nhistory.js
@@ -0,0 +1,26 @@
+var combinationSum = function (candidates, target) {
+  let output = [];
+
+  // Helper function for depth-first search
+  const dfs = (index, currentVal, arr) => {
+    // If the remaining value is less than 0, no need to proceed further
+    if (currentVal < 0) return;
+    // If we have found a valid combination
+    if (currentVal === 0) {
+      output.push([...arr]);
+      return;
+    }
+
+    // Iterate over the candidates starting from the current index
+    for (let i = index; i < candidates.length; i++) {
+      arr.push(candidates[i]);
+      dfs(i, currentVal - candidates[i], arr);
+      arr.pop(); // backtrack
+    }
+  };
+
+  // Start DFS with the initial target and empty combination
+  dfs(0, target, []);
+
+  return output;
+};
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/nhistory.js b/construct-binary-tree-from-preorder-and-inorder-traversal/nhistory.js
new file mode 100644
index 000000000..eb5bb6e84
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/nhistory.js
@@ -0,0 +1,17 @@
+var buildTree = function (preorder, inorder) {
+  // Edge case: if either preorder or inorder is empty, return null
+  if (!preorder.length || !inorder.length) return null;
+
+  // The first element in preorder is the root of the tree
+  // Find the index of the root in the inorder array
+  const root = new TreeNode(preorder[0]);
+  const mid = inorder.indexOf(root.val);
+
+  root.left = buildTree(preorder.slice(1, mid + 1), inorder.slice(0, mid));
+  root.right = buildTree(preorder.slice(mid + 1), inorder.slice(mid + 1));
+
+  return root;
+};
+
+// TC: O(n^2)
+// SC: O(n)
diff --git a/implement-trie-prefix-tree/nhistory.js b/implement-trie-prefix-tree/nhistory.js
new file mode 100644
index 000000000..ee1d8e708
--- /dev/null
+++ b/implement-trie-prefix-tree/nhistory.js
@@ -0,0 +1,51 @@
+var Trie = function () {
+  this.root = {};
+};
+
+/**
+ * @param {string} word
+ * @return {void}
+ */
+Trie.prototype.insert = function (word) {
+  let node = this.root;
+  for (let char of word) {
+    if (!node[char]) {
+      node[char] = {};
+    }
+    node = node[char];
+  }
+  node.isEndOfWord = true;
+};
+
+/**
+ * @param {string} word
+ * @return {boolean}
+ */
+Trie.prototype.search = function (word) {
+  let node = this.root;
+  for (let char of word) {
+    if (!node[char]) {
+      return false;
+    }
+    node = node[char];
+  }
+  return node.isEndOfWord === true;
+};
+
+/**
+ * @param {string} prefix
+ * @return {boolean}
+ */
+Trie.prototype.startsWith = function (prefix) {
+  let node = this.root;
+  for (let char of prefix) {
+    if (!node[char]) {
+      return false;
+    }
+    node = node[char];
+  }
+  return true;
+};
+
+// TC: O(n)
+// SC: O(n)
diff --git a/kth-smallest-element-in-a-bst/nhistory.js b/kth-smallest-element-in-a-bst/nhistory.js
new file mode 100644
index 000000000..5c90ec77e
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/nhistory.js
@@ -0,0 +1,19 @@
+var kthSmallest = function (root, k) {
+  let arr = [];
+  inOrder(root, arr);
+
+  for (let i = 0; i < arr.length; i++) {
+    if (i === k - 1) return arr[i];
+  }
+};
+
+const inOrder = (root, arr) => {
+  if (!root) return;
+
+  inOrder(root.left, arr);
+  arr.push(root.val);
+  inOrder(root.right, arr);
+};
+
+// TC: O(n)
+// SC: O(n)
diff --git a/word-search/nhistory.js b/word-search/nhistory.js
new file mode 100644
index 000000000..5c83f284e
--- /dev/null
+++ b/word-search/nhistory.js
@@ -0,0 +1,36 @@
+var exist = function (board, word) {
+  const dfs = (row, col, index) => {
+    if (word.length === index) return true;
+    if (
+      row < 0 ||
+      col < 0 ||
+      row >= board.length ||
+      col >= board[0].length ||
+      board[row][col] !== word[index]
+    )
+      return false;
+
+    board[row][col] = "#";
+    if (
+      dfs(row + 1, col, index + 1) ||
+      dfs(row, col + 1, index + 1) ||
+      dfs(row - 1, col, index + 1) ||
+      dfs(row, col - 1, index + 1)
+    )
+      return true;
+
+    board[row][col] = word[index];
+  };
+
+  for (let row = 0; row < board.length; row++) {
+    for (let col = 0; col < board[row].length; col++) {
+      if (board[row][col] === word[0] && dfs(row, col, 0)) return true;
+    }
+  }
+
+  return false;
+};
+
+// N: board column length / M: board row length / L: word length
+// TC: O(N*M*4^L)
+// SC: O(L)