剑指-从上到下打印二叉树

1、注意和前序、中序、后序遍历打印区别开来。
2、借助于队列

class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        if (!root)return result;
        que.push(root);
        while (!que.empty())
        {
            TreeNode* data = que.front();
            result.push_back(data->val);
            if (data->left != NULL)
            {
                que.push(data->left);
            }
            if (data->right != NULL)
            {
                que.push(data->right);
            }
            que.pop();
        }
        return result;
    }
private:
    vector<int>result;
    queue<TreeNode*>que;
};