diff --git a/clone-graph/nhistory.js b/clone-graph/nhistory.js
new file mode 100644
index 000000000..1cafc5dc4
--- /dev/null
+++ b/clone-graph/nhistory.js
@@ -0,0 +1,33 @@
+/**
+ * // Definition for a _Node.
+ * function _Node(val, neighbors) {
+ *    this.val = val === undefined ? 0 : val;
+ *    this.neighbors = neighbors === undefined ? [] : neighbors;
+ * };
+ */
+
+/**
+ * @param {_Node} node
+ * @return {_Node}
+ */
+var cloneGraph = function (node) {
+  let visited = {};
+
+  const dfs = (node) => {
+    if (!node) return node;
+    if (visited[node.val]) return visited[node.val];
+
+    let root = new Node(node.val);
+    visited[node.val] = root;
+
+    for (let neighbor of node.neighbors) {
+      root.neighbors.push(dfs(neighbor));
+    }
+    return root;
+  };
+
+  return dfs(node);
+};
+
+// TC: O(n+e) -> n: number of nodes | e: number of edges
+// SC: O(v) -> v: length of visited object
diff --git a/course-schedule/nhistory.js b/course-schedule/nhistory.js
new file mode 100644
index 000000000..0acf8f3f4
--- /dev/null
+++ b/course-schedule/nhistory.js
@@ -0,0 +1,49 @@
+/**
+ * @param {number} numCourses
+ * @param {number[][]} prerequisites
+ * @return {boolean}
+ */
+var canFinish = function (numCourses, prerequisites) {
+  // Initialize in-degree array and graph map
+  const inDegree = Array(numCourses).fill(0);
+  const graph = new Map();
+
+  // Build the graph and compute in-degrees
+  prerequisites.forEach(([course, pre]) => {
+    inDegree[course]++;
+    if (!graph.has(pre)) {
+      graph.set(pre, []);
+    }
+    graph.get(pre).push(course);
+  });
+
+  // Queue for courses with no prerequisites
+  const queue = [];
+  for (let i = 0; i < numCourses; i++) {
+    if (inDegree[i] === 0) {
+      queue.push(i);
+    }
+  }
+
+  // Process the courses
+  let count = 0;
+  while (queue.length > 0) {
+    const course = queue.shift();
+    count++;
+    if (graph.has(course)) {
+      graph.get(course).forEach((nextCourse) => {
+        inDegree[nextCourse]--;
+        if (inDegree[nextCourse] === 0) {
+          queue.push(nextCourse);
+        }
+      });
+    }
+  }
+
+  // Return true if all courses can be finished
+  return count === numCourses;
+};
+
+// TC: O(V + E)
+// V is the number of courses, E is the number of prerequisites.
+// SC: O(V + E)
diff --git a/design-add-and-search-words-data-structure/nhistory.js b/design-add-and-search-words-data-structure/nhistory.js
new file mode 100644
index 000000000..bdcfd2410
--- /dev/null
+++ b/design-add-and-search-words-data-structure/nhistory.js
@@ -0,0 +1,37 @@
+var WordDictionary = function () {
+  this.dictionary = new Set();
+};
+
+/**
+ * @param {string} word
+ * @return {void}
+ */
+WordDictionary.prototype.addWord = function (word) {
+  this.dictionary.add(word);
+};
+
+/**
+ * @param {string} word
+ * @return {boolean}
+ */
+WordDictionary.prototype.search = function (word) {
+  if (word.indexOf(".") != -1) {
+    // Case of word has a '.'
+    for (let str of this.dictionary) {
+      if (str.length != word.length) continue;
+      let i;
+      for (i = 0; i < word.length; i++) {
+        if (word[i] === ".") continue;
+        if (word[i] != str[i]) break;
+      }
+      if (i === str.length) return true;
+    }
+    return false;
+  } else {
+    return this.dictionary.has(word);
+  }
+};
+
+// n: number of words | m: length of the word
+// TC: O(n*m)
+// SC: O(n*m)
diff --git a/number-of-islands/nhistory.js b/number-of-islands/nhistory.js
new file mode 100644
index 000000000..694dff026
--- /dev/null
+++ b/number-of-islands/nhistory.js
@@ -0,0 +1,42 @@
+var numIslands = function (grid) {
+  // Declare row and column length
+  const m = grid.length,
+    n = grid[0].length;
+  let numIslands = 0;
+
+  // Available directions for depth-first search
+  const dir = [
+    [0, 1],
+    [1, 0],
+    [0, -1],
+    [-1, 0],
+  ];
+
+  // Function to depth-first search inside of grid
+  const dfs = (i, j) => {
+    grid[i][j] = "2";
+
+    let x, y;
+    for (d of dir) {
+      x = i + d[0];
+      y = j + d[1];
+      if (x >= 0 && x < m && y >= 0 && y < n && grid[x][y] === "1") {
+        dfs(x, y);
+      }
+    }
+    return;
+  };
+
+  for (let i = 0; i < m; i++) {
+    for (let j = 0; j < n; j++) {
+      if (grid[i][j] === "1") {
+        dfs(i, j);
+        numIslands++;
+      }
+    }
+  }
+  return numIslands;
+};
+
+// TC: O(m*n)
+// SC: O(m*n)
diff --git a/pacific-atlantic-water-flow/nhistory.js b/pacific-atlantic-water-flow/nhistory.js
new file mode 100644
index 000000000..2b6b99d07
--- /dev/null
+++ b/pacific-atlantic-water-flow/nhistory.js
@@ -0,0 +1,63 @@
+/**
+ * @param {number[][]} heights
+ * @return {number[][]}
+ */
+var pacificAtlantic = function (heights) {
+  const m = heights.length,
+    n = heights[0].length;
+  let result = [];
+
+  const pacific = new Array(m).fill(null).map(() => new Array(n).fill(false));
+  const atlantic = new Array(m).fill(null).map(() => new Array(n).fill(false));
+
+  const dir = [
+    [0, 1],
+    [1, 0],
+    [0, -1],
+    [-1, 0],
+  ];
+
+  const dfs = (i, j, ocean) => {
+    // Check visited cell
+    ocean[i][j] = true;
+
+    for (d of dir) {
+      let x = i + d[0],
+        y = j + d[1];
+      if (
+        x >= 0 &&
+        x < m &&
+        y >= 0 &&
+        y < n &&
+        !ocean[x][y] &&
+        heights[x][y] >= heights[i][j]
+      ) {
+        dfs(x, y, ocean);
+      }
+    }
+  };
+
+  // Check the cells can flow left and right edge
+  for (let i = 0; i < m; i++) {
+    dfs(i, 0, pacific);
+    dfs(i, n - 1, atlantic);
+  }
+
+  // Check the cells can flow top and bottom edge
+  for (let j = 0; j < n; j++) {
+    dfs(0, j, pacific);
+    dfs(m - 1, j, atlantic);
+  }
+
+  for (let i = 0; i < m; i++) {
+    for (let j = 0; j < n; j++) {
+      if (pacific[i][j] && atlantic[i][j]) {
+        result.push([i, j]);
+      }
+    }
+  }
+  return result;
+};
+
+// TC: O(m*n)
+// SC: O(m*n)