# 标签归档：leetcode

## 题目描述：

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

Table: Customers.

```+----+-------+
| Id | Name  |
+----+-------+
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |
+----+-------+
```

Table ...

## 题目描述：

Given a binary tree, return the postorder traversal of its nodes' values.

```For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [3,2,1].```

Note: Recursive solution is trivial, could you do it iteratively?

## 题目描述：

Given a binary tree, return the inorder traversal of its nodes' values.

```For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,3,2].```

Note: Recursive solution is trivial, could you do it iteratively?

## 题目描述：

Write a SQL query to find all duplicate emails in a table named Person.

```+----+---------+
| Id | Email   |
+----+---------+
| 1  | a@b.com |
| 2  | c@d.com |
| 3  | a@b.com |
+----+---------+
For example, your query should return the following for the above ...```

## 题目描述：

The Employee table holds all employees including their managers. Every employee has an Id, and there is also a column for the manager Id.

```+----+-------+--------+-----------+
| Id | Name  | Salary | ManagerId |
+----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000 ...```