diff --git a/doitchuu/BalancedBinaryTree.js b/doitchuu/BalancedBinaryTree.js
new file mode 100644
index 0000000..542be12
--- /dev/null
+++ b/doitchuu/BalancedBinaryTree.js
@@ -0,0 +1,29 @@
+/**
+ * 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} root
+ * @return {boolean}
+ */
+var isBalanced = function(root) {
+    function dfs(node) {
+        if (node === null) return 0;
+
+        const left = dfs(node.left);
+        if (left === -1) return -1;
+
+        const right = dfs(node.right);
+        if (right === -1) return -1;
+
+        if (Math.abs(left - right) > 1) return -1;
+
+        return Math.max(left, right) + 1;
+    }
+
+    return dfs(root) !== -1;
+};
diff --git a/doitchuu/ImplementQueueUsingStacks.js b/doitchuu/ImplementQueueUsingStacks.js
new file mode 100644
index 0000000..e17cee3
--- /dev/null
+++ b/doitchuu/ImplementQueueUsingStacks.js
@@ -0,0 +1,58 @@
+var MyQueue = function () {
+  this.inStack = [];
+  this.outStack = [];
+};
+
+/**
+ * @param {number} x
+ * @return {void}
+ */
+MyQueue.prototype.push = function (x) {
+  this.inStack.push(x);
+};
+
+/**
+ * @return {number}
+ */
+MyQueue.prototype.pop = function () {
+  if (this.outStack.length === 0) {
+    while (this.inStack.length) {
+      this.outStack.push(this.inStack.pop());
+    }
+  }
+
+  return this.outStack.pop();
+};
+
+/**
+ * @return {number}
+ */
+MyQueue.prototype.peek = function () {
+  if (this.outStack.length === 0) {
+    while (this.inStack.length) {
+      this.outStack.push(this.inStack.pop());
+    }
+  }
+
+  const lastIndex = this.outStack.length - 1;
+  return this.outStack[lastIndex];
+};
+
+/**
+ * @return {boolean}
+ */
+MyQueue.prototype.empty = function () {
+  const hasInStackValues = this.inStack.length !== 0;
+  const hasOutStackValues = this.outStack.length !== 0;
+
+  return !hasInStackValues && !hasOutStackValues ? true : false;
+};
+
+/**
+ * Your MyQueue object will be instantiated and called as such:
+ * var obj = new MyQueue()
+ * obj.push(x)
+ * var param_2 = obj.pop()
+ * var param_3 = obj.peek()
+ * var param_4 = obj.empty()
+ */
diff --git a/doitchuu/LinkedListCycle.js b/doitchuu/LinkedListCycle.js
new file mode 100644
index 0000000..d3e6af5
--- /dev/null
+++ b/doitchuu/LinkedListCycle.js
@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+
+/**
+ * @param {ListNode} head
+ * @return {boolean}
+ */
+var hasCycle = function (head) {
+  const isVisited = new Set();
+  let current = head;
+
+  while (current) {
+    if (isVisited.has(current)) {
+      return true;
+    }
+
+    isVisited.add(current);
+    current = current.next;
+  }
+
+  return false;
+};
diff --git a/doitchuu/LowestCommonAncestorOfABinarySearchTree.js b/doitchuu/LowestCommonAncestorOfABinarySearchTree.js
new file mode 100644
index 0000000..67785ad
--- /dev/null
+++ b/doitchuu/LowestCommonAncestorOfABinarySearchTree.js
@@ -0,0 +1,30 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+
+/**
+ * @param {TreeNode} root
+ * @param {TreeNode} p
+ * @param {TreeNode} q
+ * @return {TreeNode}
+ */
+var lowestCommonAncestor = function(root, p, q) {
+  const small = Math.min(p.val, q.val);
+  const big = Math.max(p.val, q.val);
+
+  while (root) {
+    if (big < root.val) {
+      root = root.left;
+    } else if (small > root.val) {
+      root = root.right;
+    } else {
+      return root;
+    }
+  }
+
+  return null;
+};