21 从上往下打印二叉树

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