diff --git a/non-overlapping-intervals/hsskey.js b/non-overlapping-intervals/hsskey.js
new file mode 100644
index 000000000..cfbc94798
--- /dev/null
+++ b/non-overlapping-intervals/hsskey.js
@@ -0,0 +1,24 @@
+/**
+ * @param {number[][]} intervals
+ * @return {number}
+ */
+var eraseOverlapIntervals = function(intervals) {
+    intervals.sort((a, b) => a[0] - b[0]);
+
+    let res = 0;
+    let prevEnd = intervals[0][1];
+
+    for (let i = 1; i < intervals.length; i++) {
+        const [start, end] = intervals[i];
+
+        if (start >= prevEnd) {
+            prevEnd = end;
+        } else {
+            res += 1;
+            prevEnd = Math.min(end, prevEnd);
+        }
+    }
+
+    return res;
+};
+
diff --git a/number-of-connected-components-in-an-undirected-graph/hsskey.js b/number-of-connected-components-in-an-undirected-graph/hsskey.js
new file mode 100644
index 000000000..b9118670d
--- /dev/null
+++ b/number-of-connected-components-in-an-undirected-graph/hsskey.js
@@ -0,0 +1,44 @@
+export class Solution {
+  /**
+   * @param {number} n - the number of vertices
+   * @param {number[][]} edges - the edges of undirected graph
+   * @return {number} - the number of connected components
+   */
+  countComponents(n, edges) {
+    const par = Array.from({ length: n }, (_, i) => i);
+    const rank = Array(n).fill(1);
+
+    const find = (n1) => {
+      let res = n1;
+      while (res !== par[res]) {
+        par[res] = par[par[res]]; // path compression
+        res = par[res];
+      }
+      return res;
+    };
+
+    const union = (n1, n2) => {
+      const p1 = find(n1);
+      const p2 = find(n2);
+      if (p1 === p2) return 0;
+
+      if (rank[p2] > rank[p1]) {
+        par[p1] = p2;
+        rank[p2] += rank[p1];
+      } else {
+        par[p2] = p1;
+        rank[p1] += rank[p2];
+      }
+
+      return 1;
+    };
+
+    let res = n;
+    for (const [n1, n2] of edges) {
+      res -= union(n1, n2);
+    }
+
+    return res;
+  }
+}
+
diff --git a/remove-nth-node-from-end-of-list/hsskey.js b/remove-nth-node-from-end-of-list/hsskey.js
new file mode 100644
index 000000000..c4383477b
--- /dev/null
+++ b/remove-nth-node-from-end-of-list/hsskey.js
@@ -0,0 +1,36 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+
+/**
+ * @param {ListNode} head
+ * @param {number} n
+ * @return {ListNode}
+ */
+var removeNthFromEnd = function(head, n) {
+    const dummy = new ListNode(0, head);
+    let left = dummy;
+    let right = head;
+
+    // advance right pointer n steps ahead
+    while (n > 0 && right) {
+        right = right.next;
+        n--;
+    }
+
+    // move both pointers until right reaches the end
+    while (right) {
+        left = left.next;
+        right = right.next;
+    }
+
+    // delete the nth node from end
+    left.next = left.next.next;
+
+    return dummy.next;
+};
+
diff --git a/same-tree/hsskey.js b/same-tree/hsskey.js
new file mode 100644
index 000000000..4fbc62a2b
--- /dev/null
+++ b/same-tree/hsskey.js
@@ -0,0 +1,24 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+
+/**
+ * @param {TreeNode} p
+ * @param {TreeNode} q
+ * @return {boolean}
+ */
+var isSameTree = function(p, q) {
+    if (!p && !q) {
+        return true;
+    }
+    if (!p || !q || p.val !== q.val) {
+        return false;
+    }
+    return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
+};
+
diff --git a/serialize-and-deserialize-binary-tree/hsskey.js b/serialize-and-deserialize-binary-tree/hsskey.js
new file mode 100644
index 000000000..f9b216290
--- /dev/null
+++ b/serialize-and-deserialize-binary-tree/hsskey.js
@@ -0,0 +1,56 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+
+/**
+ * Encodes a tree to a single string.
+ *
+ * @param {TreeNode} root
+ * @return {string}
+ */
+var serialize = function(root) {
+    const res = [];
+
+    const dfs = (node) => {
+        if (!node) {
+            res.push("N");
+            return;
+        }
+        res.push(String(node.val));
+        dfs(node.left);
+        dfs(node.right);
+    };
+
+    dfs(root);
+    return res.join(".");
+};
+
+/**
+ * Decodes your encoded data to tree.
+ *
+ * @param {string} data
+ * @return {TreeNode}
+ */
+var deserialize = function(data) {
+    const vals = data.split(".");
+    let i = 0;
+
+    const dfs = () => {
+        if (vals[i] === "N") {
+            i++;
+            return null;
+        }
+        const node = new TreeNode(parseInt(vals[i]));
+        i++;
+        node.left = dfs();
+        node.right = dfs();
+        return node;
+    };
+
+    return dfs();
+};
+