力扣：919. 完全二叉树插入器

import java.util.ArrayDeque;
import java.util.Queue;


public class Solution {
    public static void main(String[] args) {
        Solution solution = new Solution();
        CBTInserter node = new CBTInserter(new TreeNode(1, new TreeNode(2), null));
        node.insert(3);
        node.insert(4);
        node.print();
    }


}

class CBTInserter {

    Queue candidate;
    TreeNode root;
    public CBTInserter(TreeNode root) {
        this.root = root;
        candidate = new ArrayDeque<>();

        Queue queue = new ArrayDeque<>();
        queue.offer(root);
        while (!queue.isEmpty()){
            TreeNode poll = queue.poll();
            if (poll.left != null){
                queue.offer(poll.left);
            }

            if (poll.right != null){
                queue.offer(poll.right);
            }

            if (!(poll.left != null && poll.right != null)){
                candidate.offer(poll);
            }
        }
    }

    public int insert(int val) {
        TreeNode node = candidate.peek();
        TreeNode cur = new TreeNode(val);
        int res = node.val;
        if (node.left == null){
            node.left = cur;
        } else {
            node.right = cur;
            candidate.poll();
        }
        candidate.offer(cur);
        return res;
    }


    public TreeNode get_root() {
        return root;
    }

    public void print(){
        this.root.print();
    }
}

class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {}
      TreeNode(int val) { this.val = val; }
      TreeNode(int val, TreeNode left, TreeNode right) {
          this.val = val;
          this.left = left;
          this.right = right;
      }

      public void print(){
          System.out.print(this.val + " ");
          if (this.left != null){
              this.left.print();
          }
          if (this.right != null){
              this.right.print();
          }
      }
  }