剑指 Offer 32 - II. 从上到下打印二叉树 II（简单，层序遍历）

class Solution {
public:
  vector<vector<int>> levelOrder(TreeNode* root) {
    vector<vector<int>> v;
    if (root == nullptr) return v;
    queue<TreeNode*> q;
    q.push(root);
    while (!q.empty()) {
      int current_level_size = q.size();
      v.push_back(vector<int>());
      for (int i = 1; i <= current_level_size; i++) {
        auto node = q.front();
        q.pop();
        v.back().push_back(node->val);
        if (node->left) q.push(node->left);
        if (node->right) q.push(node->right);
      }
    }
    return v;
  }
};