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: introductory

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,'')
        else:
            if i.lower() not in s:
                s=s.replace(i,'')
    return s

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