题目链接：​​剑指 Offer 32 - I. 从上到下打印二叉树​​

思路：层序遍历

class Solution {
public:
  vector<int> levelOrder(TreeNode* root) {
    vector<int> v;
    if (root == nullptr) return v;

    queue<TreeNode*> q;
    q.push(root);
    while (!q.empty()) {
      int current_level_size = q.size();

      for (int i = 1; i <= current_level_size; i++) {   //等号别忘记
        auto node = q.front();
        q.pop();
        v.push_back(node->val);

        if (node->left) q.push(node->left);
        if (node->right) q.push(node->right);
      }
    }

    return v;
  }
};