void connect(TreeLinkNode *root)

      {

          while (root)

          {

              //每一层循环时重新初始化

              TreeLinkNode *prev = nullptr;

              TreeLinkNode *next = nullptr;

              //对于每一层

              for (; root; root = root->next)

              {

                  //每一层开始时，记录下一层的起始结点

                  if (!next)next = root->left ? root->left : root->right;

                  if (root->left)

                  {

                      //如果不是起始结点，则将prev与该左子结点相连接

                      if (prev)prev->next = root->left;

                      //如果是每层的起始结点，则将左子结点直接赋给prev

                      prev = root->left;

                  }

                  if (root->right)

                  {

                      if (prev)prev->next = root->right;

                      prev = root->right;

                  }

              }

              root = next;

          }

      }