题解 | #求二叉树的层序遍历#

一开始确定层数的结点个数卡住了，好好反思！

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 * };
 */

class Solution {
public:
    vector<vector<int> > levelOrder(TreeNode* root) {
      std::queue<TreeNode *> queue_;
      std::vector<std::vector<int>> res;
      
      if (root == nullptr) {
        return res;
      }
      
      queue_.push(root);
      
      while (!queue_.empty()) {
          //  获取每一层结点个数
        int size = queue_.size();
        std::vector<int> tmp;
        
        for (int i = 0; i < size; ++i) {
            //  出队的同时孩子结点入队
          TreeNode *ptr = queue_.front();
          queue_.pop();
          tmp.push_back(ptr->val);
          if (ptr->left) {
            queue_.push(ptr->left);
          }
          if (ptr->right) {
            queue_.push(ptr->right);
          }
        }
        
          //  保存每一层的结果
        res.emplace_back(std::move(tmp));
      }
      
      return res;
    }
};