LeetCode 958. Check Completeness of a Binary Tree

原題鏈接在這里：https://leetcode.com/problems/check-completeness-of-a-binary-tree/

題目：

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.

題解：

Perform level order traversal on the tree, if popped node is not null, then add its left and right child, no matter if they are null or not.

If the tree is complete, when first met null in the queue, then the rest should all be null. Otherwise, it is not complete.

Time Complexity: O(n).

Space: O(n).

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isCompleteTree(TreeNode root) {
        LinkedList<TreeNode> que = new LinkedList<TreeNode>();
        que.add(root);
        while(que.peek() != null){
            TreeNode cur = que.poll();
            que.add(cur.left);
            que.add(cur.right);
        }
        
        while(!que.isEmpty() && que.peek() == null){
            que.poll();
        }
        
        return que.isEmpty();
    }
}

類似Complete Binary Tree Inserter.