110. Balanced Binary Tree

Description

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 depth of the two subtrees of every node never differ by more than 1.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

tree

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

tree

Return false.

Solution

DFS, time O(n), space O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        
        return isBalanced(root.left) && isBalanced(root.right)
            && Math.abs(depth(root.left) - depth(root.right)) <= 1;
    }
    
    private int depth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        return 1 + Math.max(depth(root.left), depth(root.right));
    }
}