yukicoder No.39 桁の数字を入れ替え

YukiCoder36_2.nb

(* http://yukicoder.me/problems/no/39 *)

replace[li_, {posA_, posB_}] := ReplacePart[li, {posA -> li[[posB]], posB -> li[[posA]]}]

replacePos[li_] := Select[Tuples[Range[Length[li]], 2], OrderedQ]

map[li_] := replace[li, #] & /@ replacePos[li]

f[n_] := Max[FromDigits[#] & /@ map[IntegerDigits[n]]]

In[1]:= f[12]
Out[1]= 21

In[2]:= f[1122]
Out[2]= 2121

In[3]:= f[999999999]
Out[3]= 999999999

In[4]:= f[9978] 
Out[4]= 9987