110. Balanced Binary Tree

Easy

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

For this problem, 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 = [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

Approach #

class Solution {
    public boolean isBalanced(TreeNode root) {
        return getHeight(root) != -1;
    }
    
    private int getHeight(TreeNode root){
        
        if(root == null){
            return 0;
        }
        
        int heightLeftSt = getHeight(root.left);
        // can be further improved by: before recursing right, check if left is already imbalanced.
        int heightRightSt = heightLeftSt != -1 ? getHeight(root.right) : -1;
    
        // if(heightLeftSt == -1 || heightRightSt == -1 || Math.abs(heightLeftSt - heightRightSt) > 1){
        //     return -1;
        // }
        
        if(heightRightSt == -1 || Math.abs(heightLeftSt - heightRightSt) > 1){
            return -1;
        }

        return Math.max(heightLeftSt, heightRightSt) +1;
                 
    }
}

104. Maximum Depth of Binary Tree

Binary Search Tree Iterator