AcWing 45. 之字形打印二叉树（C++）- BFS

题目链接：https://www.acwing.com/problem/content/43/
题目如下：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    
    
public:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
    
    
        vector<vector<int>> result;
        if(root==NULL) return result;
        
        queue<TreeNode*> que;
        que.push(root);
        bool flag=false;
        
        while(!que.empty()){
    
    
            int size=que.size();
            vector<int> temp;
            
            while(size--){
    
    
                TreeNode* node=que.front();
                que.pop();
                
                temp.push_back(node->val);
                if(node->left) que.push(node->left);
                if(node->right) que.push(node->right);
            }
            //两者交替的处理部分
            if(flag==true){
    
    
                reverse(temp.begin(),temp.end());
                flag=false;
            }else flag=true;
            
            result.push_back(temp);
        }
        
        return result;
    }
};