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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    int maxDepth(TreeNode *root) {
		MaxDepth = 0;
		dfs(root,0);
		return MaxDepth;
    }
private:
	int MaxDepth;
	void dfs(TreeNode *root,int level)
	{
		if(root == NULL) return;
		if(root->left == NULL && root->right == NULL)
		{
			if(MaxDepth < level) MaxDepth = level;
		}
		dfs(root->left,level+1);
		dfs(root->right,level+1);
	}
};