LintCode Maximum Depth of Binary Tree

 1 /**
  * Definition of TreeNode:
  * public class TreeNode {
  *     public int val;
  *     public TreeNode left, right;
  *     public TreeNode(int val) {
  *         this.val = val;
  *         this.left = this.right = null;
  *     }
  * }
  */
 public class Solution {
     /**
      * @param root: The root of binary tree.
      * @return: An integer.
      */
     public int maxDepth(TreeNode root) {
         // write your code here
         if (root == null) {
             return 0;
         }
         int left = maxDepth(root.left);
         int right = maxDepth(root.right);
         return Math.max(left,right) + 1;
     }
 }

This question I used divide and conquer to do this question. I firstly divide this into left and right subproblems and then from the leaves to root return and +1;