Populating Next Right Pointers in Each Node

void connect(TreeLinkNode *root)
      {
          if (root == nullptr ||root->left==nullptr)return;

          root->left->next = root->right;


          //关键
          if (root->next != nullptr)
              root->right->next = root->next->left;

          connect(root->left);
          connect(root->right);

      }