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