Check whether a given Binary Tree is Complete or not 解答

Question

A complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible.

Solution 1 -- BFS

Using BFS and a flag.

public class Solution {
    public boolean checkCompleteBinaryTree(TreeNode root) {
        // BFS
        // flag whether means previous node has both children
        boolean flag = true;
        List<TreeNode> current = new ArrayList<TreeNode>();
        List<TreeNode> next;
        current.add(root);

        while (current.size() > 0) {
            next = new ArrayList<TreeNode>();
            for (TreeNode node : current) {
                if (!flag && (node.left != null || node.right != null))
                    return false;
                if (node.left == null && node.right != null)
                    return false;
                if (node.left != null) {
                    next.add(node.left);
                    if (node.right != null) {
                        next.add(node.right)
                        flag = true;
                    } else {
                        flag = false;
                    }
                }
            }
            current = next;
        }

        return true;
    }
}

Solution 2 -- Recursive

Using recursion