# Check Completeness Of A Binary Tree Problem

## Description

LeetCode Problem 958.

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 2^h nodes inclusive at the last level h.

Example 1:

``````1
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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.
``````

Example 2:

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Input: root = [1,2,3,4,5,null,7]
Output: false
Explanation: The node with value 7 isn't as far left as possible.
``````

Constraints:

• The number of nodes in the tree is in the range [1, 100].
• 1 <= Node.val <= 1000

## Sample C++ Code

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/**
* 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 seenNull = false;

while (q.size()) {
int sz = q.size();
auto node = q.front();
q.pop();
if (!node)
seenNull = true;
else {
if (seenNull)
return false;
q.push(node->left);
q.push(node->right);
}
}
return true;
}
};
``````