E
1522011523
tags: Tree, BFS
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
#### BFS
- 当出现了第一次有 null children的node的时候, 说明到了leaf level, mark flag = true;
- 自此以后，queue再不该有node再有child; queue后面出现的node的left/right child应该都是null
- 否则就是有问题, return false;
```
/*
Check a binary tree is completed or not.
A complete binary tree is a binary tree that every level is completed filled except the deepest level.
In the deepest level, all nodes must be as left as possible. See more definition
Example
1
/ \
2 3
/
4
is a complete binary.
1
/ \
2 3
\
4
is not a complete binary.
Challenge
Do it in O(n) time
Tags Expand
Binary Tree
*/
/*
Thoughts:
Do a BFS.
Once null occur, all the rest following it has to be null
*/
/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root, the root of binary tree.
* @return true if it is a complete binary tree, or false.
*/
public boolean isComplete(TreeNode root) {
if (root == null) {
return true;
}
Queue queue = new LinkedList();
queue.offer(root);
boolean flag = false;
while (!queue.isEmpty()) {
TreeNode node = queue.poll();
if (flag && (node.left != null || node.right != null)) {
return false;
}
if (node.left == null && node.right != null) {
return false;
} else if (node.left == null || node.right == null) {
flag = true;
}
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
return true;
}
}
```