C语言实现:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

int maxDepth(struct TreeNode* root) 
{
    if(NULL == root)
    {
        return 0;
    }
    
    int llen = maxDepth(root->left) + 1;
    int rlen = maxDepth(root->right) + 1;
    
    return (llen > rlen)? llen :rlen;
}

C++实现
计算二叉树的最大深度，和102题一个套路，只不过不用存储每一层的结点：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        int level = 0;
        queue<TreeNode *> queue;
        if(root) queue.push(root);
        while(!queue.empty())
        {
            level += 1;
            int n = queue.size();
            for(int i = 0;i < n;i++)
            {
                TreeNode * elem = queue.front();
                if(elem->left) queue.push(elem->left);
                if(elem->right) queue.push(elem->right);
                queue.pop();
            }
        }
        
        return level;
    }
};

运行结果:

Runtime: 8 ms, faster than 92.88% of C++ online submissions for Maximum Depth of Binary Tree.
Memory Usage: 19.2 MB, less than 16.22% of C++ online submissions for Maximum Depth of Binary Tree.

递归写法:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if(root == NULL)
            return 0;
        return 1 + max(maxDepth(root->left),maxDepth(root->right));
    }
};

运行结果:

Runtime: 8 ms, faster than 92.88% of C++ online submissions for Maximum Depth of Binary Tree.
Memory Usage: 19.6 MB, less than 16.22% of C++ online submissions for Maximum Depth of Binary Tree.

如果程序写成这样就会出错：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if(root == NULL)
            return 0;
        
        int left = maxDepth(root->left);
        int right = maxDepth(root->right);
        
        return 1 + (left > right) ? left:right;
    }
};

原因是+的优先级高，所以上面实际执行顺序是(1 + (left > right)) ? left:right;
所以在使用三目运算符时应该要特别注意这一点，当然最稳妥的方法就是对三目运算符加括号。

class Solution {
public:
    int maxDepth(TreeNode* root) {
        std::queue<TreeNode*> q;
        int max_depth = 0;
        if(!root) return max_depth;
        q.push(root);
        
        while(!q.empty())
        {
            int elem_cnt = q.size();
            for(int i = 0;i < elem_cnt;i++)
            {
                TreeNode* curr = q.front();
                q.pop();
                if(curr->left) q.push(curr->left);
                if(curr->right) q.push(curr->right);
            }
            max_depth += 1;
        } 
        return max_depth;
    }
};