题解 | #从上往下打印二叉树#

层次遍历

/*
struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};*/
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
      std::vector<int> res;
      
      if (root == nullptr) {
        return res;
      }
      
      std::queue<TreeNode *> tree;
      tree.push(root);
      
      while (!tree.empty()) {
        int size = tree.size();
        
        for (int i = 0; i < size; ++i) {
          TreeNode *tmp = tree.front();
          tree.pop();
          res.push_back(tmp->val);
          if (tmp->left) {
            tree.push(tmp->left);
          }
          if (tmp->right) {
            tree.push(tmp->right);
          }
        }
      }
      
      return res;
    }
};