剑指32-1 从上到下打印二叉树

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> levelOrder(TreeNode* root) {
13         if(root==nullptr)
14             return {};
15         queue<TreeNode*> q;
16         vector<int> ret;
17         ret.push_back(root->val);
18         if(root->left!=nullptr)
19             q.push(root->left);
20         if(root->right!=nullptr)
21             q.push(root->right);
22         while(!q.empty()){
23             int cur_size=q.size();
24             while(cur_size!=0){
25                 TreeNode* curptr=q.front();
26                 q.pop();
27                 cur_size--;
28                 ret.push_back(curptr->val);
29                 if(curptr->left!=nullptr)
30                     q.push(curptr->left);
31                 if(curptr->right!=nullptr)
32                     q.push(curptr->right);
33             }
34         }
35         return ret;
36     }
37 };