题目描述：输入一棵二叉树，求该树的深度。从根结点到叶结点依次经过的结点（含根、叶结点）形成树的一条路径，最长路径的长度为树的深度。

C++代码：

//递归写法
struct treeNode {
	int val;
	treeNode* left;
	treeNode* right;
	treeNode(int x):val(x),left(NULL),right(NULL){}
};
class Solution {
public:
    int TreeDepth(TreeNode* pRoot)
    {
        if(pRoot==NULL) return 0;
        return max(1+TreeDepth(pRoot->left),1+TreeDepth(pRoot->right));
    }
};
//非递归写法
struct treeNode {
	int val;
	treeNode* left;
	treeNode* right;
	treeNode(int x):val(x),left(NULL),right(NULL){}
};

class Solution {
public:
	int TreeDepth(treeNode* pRoot) {
		if (pRoot == NULL)return 0;
		queue<treeNode*> q;
		q.push(pRoot);
		int depth = 0;
		while (!q.empty()) {
			int size = q.size();
			depth++;
			for (int i = 0; i < size; i++) {
				treeNode* node = q.front();
				q.pop();
				if (node->left)q.push(node->left);
				if (node->right)q.push(node->right);
			}
		}
		return depth;
	}
};