Added Next_permutation.cpp

[Neilblaze]

• Implement next permutation algorithm. i.e
• Given a word w, rearrange the letters of w to construct another word s in such a way that
• s is lexicographically greater than w.
• In case of multiple possible answers, find the lexicographically smallest one.
• Example:
• ab --> ba
• bb --> bb
• hefg --> hegf
• dhck --> dhkc
• dkhc --> hcdk