Maximum Depth of Binary Tree

very easy recursion

public class Solution {
    public int maxDepth(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(root == null)
            return 0;
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}