leetcode 919. Complete Binary Tree Inserter（如何构建一个完全二叉树/C++）

问题描述：
A complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible.
Write a data structure CBTInserter that is initialized with a complete binary tree and supports the following operations:
CBTInserter(TreeNode root) initializes the data structure on a given tree with head node root;
CBTInserter.insert(int v) will insert a TreeNode into the tree with value node.val = v so that the tree remains complete, and returns the value of the parent of the inserted TreeNode;
CBTInserter.get_root() will return the head node of the tree.
解决方法：
对于CBTInserter(TreeNode root)，众所周知，如果给定一颗树，需要按层遍历的话，可以用队列来模拟。
对于CBTInserter.insert(int v)，我们可以用一个vector来模拟树，假设树的编号从根开始是1，然后逐层增加，那么你会发现，一个节点的左孩子编号是该节点编号*2， 一个节点的右孩子编号是该节点编号*2+1，反之一个节点的父亲节点编号是该节点编号/2.大家可以自行画下体会一下。
代码：

class CBTInserter {
public:
    vector<TreeNode*>a;
    CBTInserter(TreeNode* root) {
        a.clear();
        a.push_back(NULL);//为了让根节点编号为1
        queue<TreeNode*>q;
        q.push(root);
        while(!q.empty())
        {
             TreeNode *now=q.front();q.pop();
             a.push_back(now);
             if(now->left!=NULL)q.push(now->left);
             if(now->right!=NULL)q.push(now->right);
        }
    }
    
    int insert(int v) {
        TreeNode *b=new TreeNode(v);
        int len=a.size();
        if(len%2==0)
            a[len/2]->left=b;
        else a[len/2]->right=b;
        a.push_back(b);
        return a[len/2]->val;
    }
    
    TreeNode* get_root() {
        return a[1];
    }
};