110. Balanced Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its depth = 3.

/**
 * 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;
        int l = depth(root.left);
        int r = depth(root.right);
        return Math.abs(l - r) < 2 && isBalanced(root.left) && isBalanced(root.right);
    }
    public int depth(TreeNode root){
        if(root == null) return 0;
        return Math.max(depth(root.left), depth(root.right)) + 1;
    }
}

在maxDepth of BST的基础上进一步做文章。