迭代法： 

int maxDepth(TreeNode* root) {
        int maxleft = 0, maxright = 0, maxnum;
        if (root == NULL)
            return 0;
        
        maxleft  = maxDepth(root->left) + 1;
        maxright = maxDepth(root->right) + 1;
        maxnum = max(maxleft, maxright);
        
        return maxnum;
}
 

層次遍歷： 

int maxDepth(TreeNode* root) {
        if (root == NULL)
            return 0;
        
        stack<pair<TreeNode*, int>> sta;
        sta.push(pair<TreeNode*, int>(root, 1));
        
        int maxdep = 1;
        while(!sta.empty())
        {
            TreeNode* node = sta.top().first;
            int dep        = sta.top().second;
            sta.pop();
            if(node != NULL)
            {
                maxdep = max(maxdep, dep);
                sta.push(pair<TreeNode*, int>(node->left, dep + 1));
                sta.push(pair<TreeNode*, int>(node->right, dep + 1));
            }
        }
        return maxdep;
}
 

標準庫stack：

                        stack.top()

                        stack.pop()

                        stack.push()