# Leetcode SQL

626. Exchange Seats

Design a data structure that supports all following operations in *average* **O(1)** time.

`insert(val)`

: Inserts an item val to the set if not already present.`remove(val)`

: Removes an item val from the set if present.`getRandom`

: Returns a random element from current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the**same probability**of being returned.

**Example:**

// Init an empty set.

RandomizedSet randomSet = new RandomizedSet();// Inserts 1 to the set. Returns true as 1 was inserted successfully.

randomSet.insert(1);// Returns false as 2 does not exist in the set.

randomSet.remove(2);// Inserts 2 to the set, returns true. Set now contains [1,2].

randomSet.insert(2);// getRandom should return either 1 or 2 randomly.

randomSet.getRandom();// Removes 1 from the set, returns true. Set now contains [2].

randomSet.remove(1);// 2 was already in the set, so return false.

randomSet.insert(2);// Since 2 is the only number in the set, getRandom always return 2.

randomSet.getRandom();

Logic: One thing to notice: Aside from create a new variable cnt, we also create a temp table t for cnt. Additionally, Use = instead of == as it is not boolean statement.

Solution:

`select if(cnt%2=1 and id = cnt, id, if(id%2=1, id+1, id-1)) as id,`

student from seat, (select count(*) as cnt from seat) as t order by id;