从上往下打印二叉树
C++，层序遍历。
/*
struct TreeNode {
int val;
struct TreeNode *left;
struct TreeNode *right;
TreeNode(int x) :
val(x), left(NULL), right(NULL) {
}
};*/
class Solution {
public:
vector<int> PrintFromTopToBottom(TreeNode* root) {
queue<TreeNode*> q;
vector<int> ans;
if (!root) return ans;
q.push(root);
while (!q.empty()) {
TreeNode* now = q.front();
q.pop();
ans.push_back(now->val);
if (now->left)
q.push(now->left);
if (now->right)
q.push(now->right);
}
return ans;
}
};
