LF.47.Check If Binary Tree Is Completed

Check if a given binary tree is completed. A complete binary tree is one in which every level of the binary tree 
is completely filled except possibly the last level. Furthermore, all nodes are as far left as possible.

Examples

        5

      /    

    3        8

  /   

1      4

is completed.

        5

      /    

    3        8

  /           

1      4        11

is not completed.

Corner Cases

What if the binary tree is null? Return true in this case.

public class Solution {
  public boolean isCompleted(TreeNode root) {
    // Write your solution here
    if (root == null) {
        return true ;
    }
    Deque<TreeNode> queue = new LinkedList<>();
    /*flag means: 1)the curr root could not have child
    2) the node on the same level as curr could not have child
        5

      /    

    3        8

          /   

        1      4

    */
    boolean flag = false ;
    queue.offer(root);
    while(!queue.isEmpty()){
        TreeNode curr = queue.poll() ;
        if (curr.left == null) {
            flag = true ; //左右互斥 要不都没有，要不都有，不能有一个
        } else if (flag) {
            /*
        如果已经标记不能有child（被右边或者之前的node标记－说明气泡）
        但左边不为空 则退出去
            */
            return false ;
        } else{
            queue.offer(curr.left);
        }
        if (curr.right == null) {
            flag = true;
        } else if(flag){
            return false ;
        } else{
            queue.offer(curr.right) ;
        }
    }
    return true ;
  }
}

time o(n)--每一个点都要遍历 

space o(1)