递归,非常简单

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    public int maxDepth (TreeNode root) {
        // write code here
        return deepth(root);
    }

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