面试题23：从上往下打印二叉树（Leetcode-102）

vector<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int> > ret;
if(root == NULL)
{
return ret;
}

queue<TreeNode*> q;
q.push(root);

while(!q.empty())
{
int size = q.size();
vector<int> level;
for(int i=0;i<size;++i)
{
TreeNode* node = q.front();
level.push_back(node->val);
q.pop();
if(node->left)
{
q.push(node->left);
}
if(node->right)
{
q.push(node->right);
}
}
ret.push_back(level);
}
return ret;
}