Tree Maximum depth

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

Definition of a tree:

// Java
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
        left = null;
        right = null;
    }
}

// C++
class TreeNode {
public:
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

Implement the function maxDepth that returns the maximum depth of the tree.

// Java
int maxDepth(TreeNode tree)

// C++
int maxDepth(TreeNode *tree)

Example

Function maxDepth returns 3 because the longest path from the root 5 to the farthest leaf 1 contains 3 nodes.