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.

很简单的二叉树问题，使用递归来找到数最大的深度。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
int maxDepth(struct TreeNode* root) {
    if(root==NULL)return 0;
    if(root->left==NULL&&root->right==NULL)return 1;
    int leftDepth=1+maxDepth(root->left);
    int rightDepth=1+maxDepth(root->right);
    if(leftDepth>rightDepth)return leftDepth;
    else return rightDepth;
}