Next Permutation -- Math Problem

use 2 pointers:

• put first pointer i at the second last position.
• put second pt j at the last position and judge if the val at j is greater than it is at i do the while judge each time until the position of j == i.
• in each while iteration, judge if the val at j is greater than it is ati. if so, break the inner while-loop, or j--.
• if the inner while loop is done, then swap the var at i and j and break the outer while loop.
• reverse all elems from i+1 to the end;