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: 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:

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:

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.

// 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();
}

// 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;
}