// Populating Next Right Pointers in Each Node
class Solution {
public:
  void connect(TreeLinkNode *root) {
    if (! root) return;
    if (root->left)
      root->left->next = root->right;
    if (root->right && root->next)
      root->right->next = root->next->left;
    connect(root->left);
    connect(root->right);
  }
};