Pattern After Double, Reverse, and Swap in Python

Suppose we have a number n, we have ot find the nth value from the sequence. The sequence is like below −

• xxy
• xxyxxy
• yxxyxx
• xyyxyy
• xyyxyyxyyxyy
• ...

To generate the next value, we have to follow these rules, starting with xxy as the first term −

• When we are at the start of the pattern, double it(concatenate the string with itself).

• When the last operation was doubling, reverse it.

• When the last operation was reversing, exchange all xs with ys and vice versa.

• Repeat these steps.

So, if the input is like n = 5, then the output will be "yyxyyxyyxyyx"

To solve this, we will follow these steps −

• i := 0
• ret := "xxy"
• while i < n, do
  • if i mod 3 is same as 0, then
    • ret := ret + ret
  • otherwise when i mod 3 is same as 1, then
    • ret := subarray of ret from index 0 to end-1
  • otherwise,
    • new_stringy := blank string
    • for each c in ret, do
      • if c is same as "x", then
        • new_stringy := new_stringy concatenate "y"
      • otherwise,
        • new_stringy := new_stringy concatenate "x"
    • ret := new_stringy
  • i := i + 1
• return ret

Let us see the following implementation to get better understanding −

Example

 Live Demo

class Solution:
   def solve(self, s):
      i = 0
      ret = "xxy"
      while i < s:
         if i % 3 == 0:
            ret += ret
         elif i % 3 == 1:
            ret = ret[::-1]
         else:
            new_stringy = ""
            for c in ret:
               if c == "x":
                  new_stringy += "y"
               else:
                  new_stringy += "x"
               ret = new_stringy
            i += 1
      return ret
ob = Solution()
print(ob.solve(5))

Input

5

Output

yyxyyxyyxyyx