This is the simple version of Shortest Code series. If you need some challenges, please try the [challenge version](http://www.codewars.com/kata/56f928b19982cc7a14000c9d)

## Task: 

  Every uppercase letter is Father, The corresponding lowercase letters is the Son.
    
  Give you a string ```s```, If the father and son both exist, keep them. If it is a separate existence, delete them. Return the result.
    
  For example: 
    
   ```sc("Aab")``` should return ```"Aa"```
    
   ```sc("AabBc")``` should return ```"AabB"```
   
   ```sc("AaaaAaab")``` should return ```"AaaaAaa"```(father can have a lot of son)
    
   ```sc("aAAAaAAb")``` should return ```"aAAAaAA"```(son also can have a lot of father ;-)
    
    
### Series:
 - [Bug in Apple](http://www.codewars.com/kata/56fe97b3cc08ca00e4000dc9)
 - [Father and Son](http://www.codewars.com/kata/56fe9a0c11086cd842000008)
 - [Jumping Dutch act](http://www.codewars.com/kata/570bcd9715944a2c8e000009)
 - [Planting Trees](http://www.codewars.com/kata/5710443187a36a9cee0005a1)
 - [Give me the equation](http://www.codewars.com/kata/56fe9b65cc08cafbc5000de3)
 - [Find the murderer](http://www.codewars.com/kata/570f3fc5b29c702c5500043e)
 - [Reading a Book](http://www.codewars.com/kata/570ca6a520c69f39dd0016d4)
 - [Eat watermelon](http://www.codewars.com/kata/570df12ce6e9282a7d000947)
 - [Special factor](http://www.codewars.com/kata/570e5d0b93214b1a950015b1)
 - [Guess the Hat](http://www.codewars.com/kata/570ef7a834e61306da00035b)
 - [Symmetric Sort](http://www.codewars.com/kata/5705aeb041e5befba20010ba)
 - [Are they symmetrical?](http://www.codewars.com/kata/5705cc3161944b10fd0004ba)
 - [Max Value](http://www.codewars.com/kata/570771871df89cf59b000742)
 - [Trypophobia](http://www.codewars.com/kata/56fe9ffbc25bf33fff000f7c)
 - [Virus in Apple](http://www.codewars.com/kata/5700af83d1acef83fd000048)
 - [Balance Attraction](http://www.codewars.com/kata/57033601e55d30d3e0000633)
 - [Remove screws I](http://www.codewars.com/kata/5710a50d336aed828100055a)
 - [Remove screws II](http://www.codewars.com/kata/5710a8fd336aed00d9000594)
 - [Regular expression compression](http://www.codewars.com/kata/570bae4b0237999e940016e9)
 - [Collatz Array(Split or merge)](http://www.codewars.com/kata/56fe9d579b7bb6b027000001)
 - [Tidy up the room](http://www.codewars.com/kata/5703ace6e55d30d3e0001029)
 - [Waiting for a Bus](http://www.codewars.com/kata/57070eff924f343280000015)

Difficulty: EASY
Tags: Game theory

Ad-hoc

Starter code:
def sc(s):
	

Solutions:
def sc(strng):
	seen = set(strng)
	return ''.join((a for a in strng if a.swapcase() in seen))


def sc(s):
	return ''.join((ch for ch in s if ch.upper() in s and ch.lower() in s))


def sc(s):
	xs = set(s)
	return ''.join((c for c in s if c.swapcase() in xs))


def sc(s):
	return ''.join([e for e in s if {1: e.upper() in s, 0: e.lower() in s}[e.islower()]])


def sc(s):
	return ''.join((i for i in s if i.swapcase() in set(s)))


def sc(s):
	return ''.join([i for i in s if i.lower() in s and i.upper() in s])


def sc(s):
	return ''.join((c for c in s if c.swapcase() in s))


def sc(s):
	for i in s:
		if i.islower() == True:
			if i.upper() not in s:
				s = s.replace(i, '')
		elif i.lower() not in s:
			s = s.replace(i, '')
	return s