22.从上往下打印二叉树

1.思路：

运用辅助队列q，实现树的层次遍历

2.代码：

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