Balanced Binary Tree

February 8th, 2026

Description

Given a binary tree, determine if it is height-balanced.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: true

Example 2:

Input: root = [1,2,2,3,3,null,null,4,4]
Output: false

Example 3:

Input: root = []
Output: true

Constraints:

The number of nodes in the tree is in the range [0, 5000].
-10⁴ <= Node.val <= 10⁴

Solutions

This solution uses the same sentinel value approach but with more explicit variable names (l and r instead of inline expressions) and checks, making the logic slightly more readable. The dfs function short-circuits by checking if the left subtree is unbalanced before processing the right, then validates that the height difference doesn't exceed 1; overall, it achieves the same efficiency as the optimized versions but prioritizes code clarity.

Language: javascript(2026-02-08 10:43)DONE

CPU Performance100.00%

Memory Performance16.23%

/**
 * 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) {
    const dfs = (node) => {
        if (!node) return 0;

        const l = dfs(node.left);
        if (l === -1) return -1;

        const r = dfs(node.right);
        if (r === -1) return -1;

        const diff = Math.abs(r - l);
        if (diff > 1) return -1;

        return Math.max(l, r) + 1;
    };

    return dfs(root) !== -1;
};

Description

Solutions

AI Explanation