[leetcode] problem 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 2^h nodes inclusive at the last level h.

Example

No.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.

No.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

Code

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public class  {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

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public boolean isCompleteTree(TreeNode root) {
    boolean isNull = false;
    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);

    while (!queue.isEmpty()) {
        TreeNode node = queue.poll();

        if (node == null)
            isNull = true;
        else {
            if (isNull)
                return false;

            queue.offer(node.left);
            queue.offer(node.right);
        }
    }

    return true;
}