110. Balanced Binary Tree

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]:

    3
   / 
  9  20
    /  
   15   7

Return true.

Example 2:

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

       1
      / 
     2   2
    / 
   3   3
  / 
 4   4

平衡二叉搜索树（Self-balancing binary search tree）又被称为AVL树（有别于AVL算法），且具有以下性质：

它是一 棵空树或它的左右两个子树的高度差的绝对值不超过1，并且左右两个子树都是一棵平衡二叉树。

 public bool IsBalanced(TreeNode root)
        {
            if (root == null)
            {
                return true;
            }
            int maxLeftDepth = MaxDepth(root.left);
            int maxRightDepth = MaxDepth(root.right);
            bool flag1 = Math.Abs(maxLeftDepth - maxRightDepth) <= 1;
            bool flag2 = IsBalanced(root.left);
            bool flag3 = IsBalanced(root.right);
            return flag1 && flag2 && flag3;
        }

        public int MaxDepth(TreeNode root)
        {
            int depth;
            if (root == null)
            {
                depth = 0;
            }
            else
            {
                depth = 1;
                TreeNode left = root.left;
                TreeNode right = root.right;
                if (left != null || right != null)
                {
                    int leftDepth = MaxDepth(left);
                    int rightDepth = MaxDepth(right);
                    depth = depth + Math.Max(leftDepth, rightDepth);
                }
            }

            return depth;
        }