919. Complete Binary Tree Inserter

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.

Design an algorithm to insert a new node to a complete binary tree keeping it complete after the insertion.

Implement the CBTInserter class:

• CBTInserter(TreeNode root) Initializes the data structure with the root of the complete binary tree.
• int insert(int v) Inserts a TreeNode into the tree with value Node.val == val so that the tree remains complete, and returns the value of the parent of the inserted TreeNode.
• TreeNode get_root() Returns the root node of the tree.

Example 1:

Input
["CBTInserter", "insert", "insert", "get_root"]
[[[1, 2]], [3], [4], []]
Output
[null, 1, 2, [1, 2, 3, 4]]

Explanation
CBTInserter cBTInserter = new CBTInserter([1, 2]);
cBTInserter.insert(3);  // return 1
cBTInserter.insert(4);  // return 2
cBTInserter.get_root(); // return [1, 2, 3, 4]

Constraints:

• The number of nodes in the tree will be in the range [1, 1000].
• 0 <= Node.val <= 5000
• root is a complete binary tree.
• 0 <= val <= 5000
• At most 104 calls will be made to insert and get_root.

题目：实现一个类，可以往二叉树中插入节点，使得二叉树保持完全二叉树

思路：容易错的点是初始化时不只是一个根节点，可能已经是一棵树。用queue来保存所有不完全的子节点。代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class CBTInserter {
private:
    queue q;
    TreeNode* rootNode;
public:
    CBTInserter(TreeNode* root) {
        rootNode = root;
        q.push(root);
        while(!q.empty()){
            if(q.front()->left) q.push(q.front()->left);
            if(q.front()->right) q.push(q.front()->right);
            if(q.front()->left && q.front()->right)
                q.pop();
            else break;
        }
    }
    
    int insert(int val) {
        TreeNode* node = new TreeNode(val);
        int res = q.front()->val;
        if(!q.front()->left){
            q.front()->left = node;
        } else {
            q.front()->right = node;
            q.pop();
        }
        q.push(node);
        return res;
    }
    
    TreeNode* get_root() {
        return rootNode;
    }
};
 
/**
 * Your CBTInserter object will be instantiated and called as such:
 * CBTInserter* obj = new CBTInserter(root);
 * int param_1 = obj->insert(val);
 * TreeNode* param_2 = obj->get_root();
 */