题干：

力扣这里给了定义：二叉树的最大深度指的是从根节点开始，到最远叶子所经过的节点数。

代码：

class Solution {//递归实现
public:
    int maxDepth(TreeNode* root) {
        if(root == NULL)return NULL;
        int leftheight = maxDepth(root -> left);
        int rightheight = maxDepth(root -> right);
        int height = 1 + max(leftheight, rightheight);

        return height;
    }
};

class Solution {//迭代实现
public:
    int maxDepth(TreeNode* root) {
        int count = 0;
        queue<TreeNode*> que;
        vector<vector<int>> res;
        if(root != NULL) que.push(root);
        while(!que.empty()){
            int size = que.size();
            vector<int> tmp;
            while(size--){
                TreeNode* node = que.front();
                que.pop();
                tmp.push_back(node -> val);
                if(node -> left) que.push(node -> left);
                if(node -> right) que.push(node -> right);
                if(size == 0) count+=1;
            }
        }
        return count;

    }
};