102. 二叉树的层次遍历

vector<vector<int>> levelOrder(TreeNode* root) {//數據結構的層次遍歷
      vector<vector<int>> res;
      if (root == NULL) return res;
      queue<TreeNode*> q;    //指針隊列
      q.push(root);
      while (!q.empty())
      {
          int size = q.size();
          vector<int> tem;
          while (size--)
          {
              TreeNode* tree = q.front();//為頭指針
              tem.push_back(q.front()->val);
              q.pop();
              if (tree->left){
                  q.push(tree->left);
              }

              if (tree->right){
                  q.push(tree->right);
              }
        
          }
          res.push_back(tem);
      }
      return res;
  }//層次遍歷的主要思想：頭指針先入隊列，如果對壘不空，訪問頭節點。如果左子樹不空，進對列。如果右子樹不空，進隊列。