题目描述：给定一个二叉树，找出其最大深度。二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。

        编码实现：

public int maxDepth(TreeNode root) {
        if (null == root){
            return 0;
        }
        int leftDepth = maxDepth(root.left),rightDepth = maxDepth(root.right);
        return 1+(leftDepth>rightDepth?leftDepth:rightDepth);
    }

    class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {}
      TreeNode(int val) { this.val = val; }
      TreeNode(int val, TreeNode left, TreeNode right) {
          this.val = val;
          this.left = left;
          this.right = right;
      }
    }