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