/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isCompleteTree = function(root) {
  const queue = [root];
  let foundNull = false;

  while (queue.length) {
    const node = queue.shift();
    if (!node) {
      foundNull = true;
      continue;
    }
    if (foundNull) return false;
    queue.push(node.left);
    queue.push(node.right);
  }

  return true;
};