Formatted question description:

919. Complete Binary Tree Inserter (Medium)

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.


Example 1:

Input: inputs = ["CBTInserter","insert","get_root"], inputs = [[[1]],[2],[]]
Output: [null,1,[1,2]]

Example 2:

Input: inputs = ["CBTInserter","insert","insert","get_root"], inputs = [[[1,2,3,4,5,6]],[7],[8],[]]
Output: [null,3,4,[1,2,3,4,5,6,7,8]]



  1. The initial given tree is complete and contains between 1 and 1000 nodes.
  2. CBTInserter.insert is called at most 10000 times per test case.
  3. Every value of a given or inserted node is between 0 and 5000.




Related Topics:

Solution 1.

// OJ:
// Time:
//     CBTInserter: O(N)
//     insert: O(1)
//     get_root: O(1)
// Space: O(N)
class CBTInserter {
    TreeNode *root;
    queue<TreeNode*> q;
    CBTInserter(TreeNode* root): root(root) {
        if (!root) return;
        queue<TreeNode*> tmp;
        while (tmp.size()) {
            root = tmp.front();
            if (!root->left || !root->right) q.push(root);
            if (root->left) tmp.push(root->left);
            if (root->right) tmp.push(root->right);
    int insert(int v) {
        TreeNode *node = new TreeNode(v);
        TreeNode *parent = q.front();
        if (!parent->left) parent->left = node;
        else {
            parent->right = node;
        return parent->val;
    TreeNode* get_root() {
        return root;

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