Add balanced binary tree JS LC medium

Author: nwthomas

src/leetcode/medium/balanced-binary-tree/balancedBinaryTree.js

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+/*
+https://leetcode.com/problems/balanced-binary-tree
+
+Given a binary tree, return true if it is height-balanced and false otherwise.
+
+A height-balanced binary tree is defined as a binary tree in which the left and right subtrees of every node differ in height by no more than 1.
+
+Example 1:
+Input: root = [1,2,3,null,null,4]
+Output: true
+
+Example 2:
+Input: root = [1,2,3,null,null,4,null,5]
+Output: false
+
+Example 3:
+Input: root = []\
+Output: true
+
+Constraints:
+The number of nodes in the tree is in the range [0, 1000].
+-1000 <= Node.val <= 1000
+*/
+
+class TreeNode {
+    constructor(val = 0, left = null, right = null) {
+        this.val = val;
+        this.left = left;
+        this.right = right;
+    }
+}
+
+function isBalanced(root) {
+    function traverse(node) {
+        if (!node) {
+            return [true, 0];
+        }
+
+        const left = traverse(node.left);
+        const right = traverse(node.right);
+        const balanced =
+            left[0] && right[0] && Math.abs(left[1] - right[1]) <= 1;
+
+        return [balanced, 1 + Math.max(left[1], right[1])];
+    }
+
+    return traverse(root)[0];
+}
+
+module.exports = { isBalanced, TreeNode };

src/leetcode/medium/balanced-binary-tree/balancedBinaryTree.test.js

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+const { isBalanced, TreeNode } = require("./balancedBinaryTree");
+
+describe("isBalanced", () => {
+    it("should return true if the tree is balanced", () => {
+        const root = new TreeNode(1);
+        root.left = new TreeNode(2);
+        root.right = new TreeNode(3);
+        expect(isBalanced(root)).toBe(true);
+    });
+
+    it("should return false if the tree is not balanced", () => {
+        const root = new TreeNode(1);
+        root.left = new TreeNode(2);
+        root.right = new TreeNode(3);
+        root.left.left = new TreeNode(4);
+        root.left.left.left = new TreeNode(5);
+        expect(isBalanced(root)).toBe(false);
+    });
+
+    it("should return true if the tree is empty", () => {
+        const root = null;
+        expect(isBalanced(root)).toBe(true);
+    });
+});