剑指offer32-I.从上到下打印二叉树（一维数组输出）

class Solution {
public:
    vector<int> levelOrder(TreeNode* root) {
        vector<int> ans;
        if (root == NULL) return ans;
        queue<TreeNode*> q;
        q.push(root);
        ans.push_back(root->val);
        while (!q.empty()) {
            auto p = q.front();
            if (p->left) {
                q.push(p->left);
                ans.push_back(p->left->val);
            }
            if (p->right) {
                q.push(p->right);
                ans.push_back(p->right->val);
            }
            q.pop(); 
        }
        return ans;
    }
};