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]]

Note:

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.

Solution 2: Deque

Using a deck to keep track of insertable nodes (potential parents) in order.

Time complexity: O(1) / O(n) first call

Space complexity: O(n)

// Author: Huahua
class CBTInserter {
public:
  CBTInserter(TreeNode* root): root_(root), d_({root}) {}

  int insert(int v) {    
    while (!d_.empty()) {
      TreeNode* r = d_.front();
      if (r->left && r->right) {
        d_.push_back(r->left);
        d_.push_back(r->right);
        d_.pop_front();
      } else if (r->left) {
        r->right = new TreeNode(v);
        return r->val;
      } else {
        r->left = new TreeNode(v);        
        return r->val;
      }
    }
  }

  TreeNode* get_root() { return root_; }
private:
  std::deque<TreeNode*> d_;
  TreeNode* root_;
};

The post 花花酱 LeetCode 919. Complete Binary Tree Inserter appeared first on Huahua's Tech Road.