Given a binary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

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;
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}

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;
        if (root.left == null) {
            return maxDepth(root.right) + 1;
        } else if (root.right == null) {
            return maxDepth(root.left) + 1;
        } else {
            return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
        }
    }
}

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """

        if root is None:
            return 0

        left_dep = self.maxDepth(root.left)
        right_dep = self.maxDepth(root.right)
        return max(left_dep, right_dep) + 1