Find next higher number with same digits.
Example 1 : if num = 25468, o/p = 25486
Example 2 : if num = 21765, o/p = 25167
Example 3 : If num = 54321, o/p = 54321 (cause it's not possible to gen a higher num than tiz with given digits ).
Google的面试题，实际上就是next_permutation。