题解 | #从上往下打印二叉树#

保存每一层存在的节点在queue里面，依次判断打印，

/*
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> ret;
        if(!root) return ret;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            TreeNode* node=q.front();
            q.pop();
            ret.push_back(node->val);

            if(node->left) q.push(node->left);
            if(node->right) q.push(node->right);
        }
        return ret;
    }
};