## Quiz 1

There is a table World

    +-----------------+------------+------------+--------------+---------------+
    | name            | continent  | area       | population   | gdp           |
    +-----------------+------------+------------+--------------+---------------+
    | Afghanistan     | Asia       | 652230     | 25500100     | 20343000      |
    | Albania         | Europe     | 28748      | 2831741      | 12960000      |
    | Algeria         | Africa     | 2381741    | 37100000     | 188681000     |
    | Andorra         | Europe     | 468        | 78115        | 3712000       |
    | Angola          | Africa     | 1246700    | 20609294     | 100990000     |
    +-----------------+------------+------------+--------------+---------------+
A country is big if it has an area of bigger than 3 million square km or a population of more than 25 million. _(595. Big Countries)_

Write a SQL solution to output big countries' name, population and area.
For example, according to the above table, we should output:

    +--------------+-------------+--------------+
    | name         | population  | area         |
    +--------------+-------------+--------------+
    | Afghanistan  | 25500100    | 652230       |
    | Algeria      | 37100000    | 2381741      |
    +--------------+-------------+--------------+
    
## Quiz 2

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. _(183. Customers Who Never Order)_

Table: Customers.

    +----+-------+
    | Id | Name  |
    +----+-------+
    | 1  | Joe   |
    | 2  | Henry |
    | 3  | Sam   |
    | 4  | Max   |
    +----+-------+
Table: Orders.

    +----+------------+
    | Id | CustomerId |
    +----+------------+
    | 1  | 3          |
    | 2  | 1          |
    +----+------------+
Using the above tables as example, return the following:

    +-----------+
    | Customers |
    +-----------+
    | Henry     |
    | Max       |
    +-----------+
    
## Quiz 3

You're given strings J representing the types of stones that are jewels, and S representing the stones you have.  Each character in S is a type of stone you have.  You want to know how many of the stones you have are also jewels.

The letters in J are guaranteed distinct, and all characters in J and S are letters. Letters are case sensitive, so "a" is considered a different type of stone from "A". _(771. Jewels and Stones)_

### Test Case 1:
    Input: J = "aA", S = "aAAbbbb"
    Output: 3
### Test Case 2:
    Input: J = "z", S = "ZZ"
    Output: 0
### Note:
* S and J will consist of letters and have length at most 50.
* The characters in J are distinct.

### Example Code (Ruby):
    # @param {String} j
    # @param {String} s
    # @return {Integer}
    def num_jewels_in_stones(j, s)
    end  
    
## Quiz 4

Initially, there is a Robot at position (0, 0). Given a sequence of its moves, judge if this robot makes a circle, which means it moves back to the original place.

The move sequence is represented by a string. And each move is represent by a character. The valid robot moves are R (Right), L (Left), U (Up) and D (down). The output should be true or false representing whether the robot makes a circle. _(657. Judge Route Circle)_

### Test Case 1:
    Input: "UD"
    Output: true

### Test Case 2:
    Input: "LL"
    Output: false
    
### Example Code (Ruby):
    # @param {String} moves
    # @return {Boolean}
    def judge_circle(moves)
    end