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
     */
    public static boolean isCompleteTree(TreeNode root) {
        // write code here
        if (root == null || (root.right == null && root.left == null)) {
            return true;
        }
        if (depth(root.left) - depth(root.right) > 1 ||
                depth(root.left) < depth(root.right)) {
            return false;
        } else {
            if (isFull(root.left)) {
                return isComplete(root.right);
            } else {
                if (depth(root.left) == depth(root.right)) {
                    return false;
                } else {
                    return isComplete(root.left);
                }
            }
        }

    }

    public static int depth(TreeNode root) {
        if (root == null) {
            return 0;
        } else {
            return Math.max(depth(root.left), depth(root.right)) + 1;
        }
    }

    public static boolean isFull(TreeNode root) {
        if (depth(root.left) == depth(root.right))
            return true;
        return false;
    }

    public static boolean isComplete(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (root.right != null && root.left == null) {
            return false;
        }
        return isComplete(root.left) && isComplete(root.right);
    }
}