Problem: Find Permutation
Given a positive integer n and a string s consisting only of letters D or I, you have to find any permutation of first n positive integer that satisfy the given input string.
D means the next number is smaller, while I means the next number is greater.
Length of given string s will always equal to n - 1
Your solution should run in linear time and space.
Input 1: n = 3 s = ID Return: [1, 3, 2]