題目：

給定一個二叉樹，找出其最大深度。

二叉樹的深度為根節點到最遠葉子節點的距離。

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

3.二叉樹的最大深度