/*
  Given the root of a binary tree, determine if it is a complete binary tree.

  In a complete binary tree, every level, except possibly the last, is completely filled,
  and all nodes in the last level are as far left as possible. It can have between 1 and 2h
  nodes inclusive at the last level h.

  Ex. Input: root = [1,2,3,4,5,6]
      Output: true
      Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), 
      and all nodes in the last level ({4, 5, 6}) are as far left as possible.

  Time  : O(N);
  Space : O(N);
*/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isCompleteTree(TreeNode* root) {
        queue <TreeNode*> q;
        q.push(root);
        bool isNull = false;

        while(!q.empty()) {
            TreeNode * front = q.front();
            if(!front)
                isNull = true;
            else {
                if(isNull)
                    return false;
                q.push(front -> left);
                q.push(front -> right);
            }
            q.pop();
        }
        return true;
    }
};