Acwing43 不分行从上往下打印二叉树

地址 https://www.acwing.com/problem/content/description/41/

从上往下打印出二叉树的每个结点，同一层的结点按照从左到右的顺序打印。

样例

输入如下图所示二叉树[, , , null, null, , null, , null, null, null]

   / \

     /

   /

输出：[, , , , ]

bfs典型例题

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> res;
    queue<TreeNode* > que;

    void bfs()
    {
        while(!que.empty()){
            TreeNode* p = que.front();
            que.pop();
            res.push_back(p->val);
            if(p->left != NULL)
                que.push(p->left);
            if(p->right != NULL)
                que.push(p->right);
        }
    }

    vector<int> printFromTopToBottom(TreeNode* root) {
        if(root == NULL) return res;
        que.push(root);
        bfs();

        return res;
    }
};