> For the complete documentation index, see [llms.txt](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/tree/958.-check-completeness-of-a-binary-tree.md).

# 958. Check Completeness of a Binary Tree

Given a binary tree, determine if it is a *complete binary tree*.

**Definition of a complete binary tree from** [**Wikipedia**](http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees)**:**\
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.

**Example 1:**

![](https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-1.png)

```
Input: [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:**

![](https://assets.leetcode.com/uploads/2018/12/15/complete-binary-tree-2.png)

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

**Note:**

1. The tree will have between 1 and 100 nodes.

```cpp
// BFS Level Order Traversal
bool isCompleteTree(TreeNode* root) { // time: O(n); space: O(n)
    vector<TreeNode*> nodes({root});
    int i = 0;
    while (i < nodes.size() && nodes[i]) {
        nodes.push_back(nodes[i]->left);
        nodes.push_back(nodes[i]->right);
        ++i;
    }
    while (i < nodes.size() && !nodes[i]) {
        ++i;
    }
    return i == nodes.size();
}
```

```cpp
// BFS Level Order Traversal
bool isCompleteTree(TreeNode* root) { // time: O(n); space: O(n)
    bool end = false;
    queue<TreeNode*> q({root});
    while (!q.empty()) {
        TreeNode* t = q.front(); q.pop();
        if (!t) end = true;
        else {
            if (end) return false;
            q.push(t->left);
            q.push(t->right);
        }
    }
    return true;
}
```