面试题32 - I. 从上到下打印二叉树

题目：

 

 

解答：

/**
 * 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 (NULL == root)
        {
            return res;
        }

        queue<TreeNode*> q;
        q.push(root);
        int count = 1;
        int level = 0;
        
        while (!q.empty())
        {
            level = 0;
            for (int i = 0; i < count; i++)
            {
                TreeNode *tmp = q.front();
                q.pop();
                res.push_back(tmp->val);

                if (tmp->left)
                {
                    q.push(tmp->left);
                    level++;
                }
                if (tmp->right)
                {
                    q.push(tmp->right);
                    level++;
                }
            }

            count = level;
        }

        return res;
    }
};