LeetCode 919. Complete Binary Tree Inserter

原题链接在这里：https://leetcode.com/problems/complete-binary-tree-inserter/

题目：

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.

题解：

Do level order traversal on the tree. Keep the nodes that doesn't have both left and right child in the queue.

Pop the first node in the queue and mark it as current.

When inserting, check current node's left, if null, insert it as left child. Or check current node's right, if null, insert it as right child. If both not null, then pop another node from queue and insert it as left child.

Time Complexity: CBTInserter, O(n). insert, O(1). get_root, O(1).

Space: O(n). queue space.

AC Java:

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 class CBTInserter {
     TreeNode root;
     TreeNode cur;
     LinkedList<TreeNode> que;
     public CBTInserter(TreeNode root) {
         this.root = root;
         que = new LinkedList<TreeNode>();
         que.add(root);
         while(que.peek().left != null && que.peek().right != null){
             TreeNode top = que.poll();
             que.add(top.left);
             que.add(top.right);
         }

         cur = que.poll();
         if(cur.left != null){
             que.add(cur.left);
         }
     }

     public int insert(int v) {
         TreeNode newNode = new TreeNode(v);
         que.add(newNode);

         if(cur.left == null){
             cur.left = newNode;
         }else if(cur.right == null){
             cur.right = newNode;
         }else{
             cur = que.poll();
             cur.left = newNode;
         }

         return cur.val;
     }

     public TreeNode get_root() {
         return this.root;
     }
 }

 /**
  * Your CBTInserter object will be instantiated and called as such:
  * CBTInserter obj = new CBTInserter(root);
  * int param_1 = obj.insert(v);
  * TreeNode param_2 = obj.get_root();
  */

类似Check Completeness of a Binary Tree.