How to determine whether a binary tree is complete?

Question

A complete binary tree is defined as a binary tree in which every level, except possibly the deepest, is completely filled. At deepest level, all nodes must be as far left a

Answer 1

private static boolean isCompleteBinaryTree(TreeNode root) {

if (root == null) {
    return false;
} else {
    boolean completeFlag = false;
    List list = new ArrayList();
    list.add(root);
    while (!list.isEmpty()) {
        TreeNode element = list.remove(0);
        if (element.left != null) {
            if (completeFlag) {
                return false;
            }
        list.add(element.left);
        } else {
            completeFlag = true;
        }
        if (element.right != null) {
            if (completeFlag) {
                return false;
            }
        list.add(element.right);
        } else {
            completeFlag = true;
        }
    }
        return true;
    }
}

Reference: Check the following link for a detailed explanation http://www.geeksforgeeks.org/check-if-a-given-binary-tree-is-complete-tree-or-not/