剑指offer 二叉平衡树

双重递归（利用上一题的结果）

public class Solution {
    public int TreeDepth(TreeNode root) {
        if (root == null) return 0;
        return 1 + Math.max(TreeDepth(root.left), TreeDepth(root.right));
    }

    public boolean IsBalanced_Solution(TreeNode root) {
        if (root == null) return true;
        return Math.abs(TreeDepth(root.left) - TreeDepth(root.right)) <= 1
            && IsBalanced_Solution(root.left)
            && IsBalanced_Solution(root.right);
    }
}