剑指offer--29.从上往下打印二叉树

层序遍历，队列

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时间限制：1秒 空间限制：32768K 热度指数：314273

 算法知识视频讲解

题目描述

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

/*
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>  que;
        queue<TreeNode*>Q;
        TreeNode* start;
        if(root == NULL) return que;
        Q.push(root);
        while(!Q.empty())
        {
        start=Q.front();
        que.push_back(start->val);
        if(start->left != NULL)
            Q.push(start->left);
        if(start->right != NULL)
            Q.push(start->right);
        Q.pop();
        }
 
       return que;
    }
};