题解 | #判断是不是完全二叉树#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return bool布尔型
     */
    // 用来标记是否出现过非双全的节点
    boolean flag = false;
    public boolean isCompleteTree (TreeNode root) {
        // write code here
        if(root == null) return true;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()) {
            TreeNode node = q.poll();
            TreeNode left = node.left;
            TreeNode right = node.right;
            if((flag && !(left == null && right == null)) || (left == null && right != null)) {
                return false;
            }
            if(left == null || right == null) {
                flag = true;
            }
            if(left != null) q.offer(left);
            if(right != null) q.offer(right); 
        }
        return true;
    }
}