剑指 Offer 32 - I. 从上到下打印二叉树

从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。

示例：
二叉树：[3,9,20,null,null,15,7],

        3
   / \
  9  20
    /  \
   15   7

返回结果：

[3,9,20,15,7]

/**
 * 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> levelOrder(TreeNode* root) {
        vector<int> res;
        if(root == NULL) return res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            TreeNode* cur = q.front();
            q.pop();
            res.push_back(cur->val);
            if(cur->left != NULL) q.push(cur->left);
            if(cur->right != NULL) q.push(cur->right);
        }
        return res;
    }
};