typedef struct TNode {  int data;  TNode* left;  TNode* right;  TNode* next;  };  //时间复杂度为O(n)，空间复杂度为O(n)  void addNext(TNode* root) {  if (!root) {  return;  }  queue<TNode*> q;  q.push(root);  while (!q.empty()) {  int levelLength = q.size();  TNode* first = NULL;  TNode* second = NULL;  while (levelLength) {  first = q.front();  q.pop();  if (first->left) {  q.push(first->left);  }  if (first->right) {  q.push(first->right);  }  if (--levelLength == 0) {  break;  }  second = q.front();  first->next = second;  }  }  }