958. Check Completeness of a Binary Tree
看到第一个 node 不为空，之后所有的 node 只要有一个为空就 return false
/**
* 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) {
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
// If the flag is set to true, there should not be any child nodes afterwards
boolean flag = false;
while(!queue.isEmpty()) {
TreeNode cur = queue.poll();
if (cur.left == null) {
flag = true;
} else if (flag) {
return false;
} else {
queue.offer(cur.left);
}
if (cur.right == null) {
flag = true;
} else if (flag) {
return false;
} else {
queue.offer(cur.right);
}
}
return true;
}
}