Swap adjacent in LR string

Time: O(N); Space: O(1); medium

In a string composed of ‘L’, ‘R’, and ‘X’ characters, like “RXXLRXRXL”, a move consists of either replacing one occurrence of “XL” with “LX”, or replacing one occurrence of “RX” with “XR”.

Given the starting string start and the ending string end, return True if and only if there exists a sequence of moves to transform one string to the other.

Example 1:

Input: start = “RXXLRXRXL”, end = “XRLXXRRLX”

Output: True

Explanation:

  • We can transform start to end following these steps:

    • RXXLRXRXL ->

    • XRXLRXRXL ->

    • XRLXRXRXL ->

    • XRLXXRRXL ->

    • XRLXXRRLX

Example 2:

Input: start = “X”, end = “L”

Ouput: False

Constraints:

  • 1 <= len(start) == len(end) <= 10000.

  • Both start and end will only consist of characters in {‘L’, ‘R’, ‘X’}.

[2]:

class Solution1(object):
    """
    Time: O(N)
    Space: O(1)
    """
    def canTransform(self, start, end):
        """
        :type start: str
        :type end: str
        :rtype: bool
        """
        N = len(start)
        i, j = 0, 0

        while i < N and j < N:

            while i < N and start[i] == 'X':
                i += 1

            while j < N and end[j] == 'X':
                j += 1

            if (i < N) != (j < N):
                return False

            elif i < N and j < N:
                if start[i] != end[j] or \
                   (start[i] == 'L' and i < j) or \
                   (start[i] == 'R' and i > j):
                    return False
            i += 1
            j += 1

        return True

[3]:

s = Solution1()

start = "RXXLRXRXL"
end = "XRLXXRRLX"
assert s.canTransform(start, end) == True

start = "X"
end = "L"
assert s.canTransform(start, end) == False