#include <queue>
#include <cstdio>

typedef struct TreeNode {
  struct TreeNode *l;
  struct TreeNode *r;
  int val;
} TreeNode;

// answer begins here
void BFS(TreeNode *node) {
  std::queue<TreeNode *> todo;
  if (node) {
    todo.push(node);
  }

  while (!todo.empty()) {
    for (int i = 0, size = todo.size(); i < size; i++) {
      auto ptr = todo.front();
      std::printf("%d ", ptr->val);
      todo.pop();

      if (ptr->l) {
        todo.push(ptr->l);
      }
      if (ptr->r) {
        todo.push(ptr->r);
      }
    }
    printf("\n");
  }
}

// answer ends
int main(void) {
  BFS(nullptr);
  return 0;
}