Backspace String Compare

backspace_string_compare.py

class Solution:
    def backspaceCompare(self, S: str, T: str) -> bool:
        def next_index(R: str, i: int) -> int:
            """
            Get the index of the next undeleted letter

            @param R: str string
            @param i: index
            @return: current letter index
            """
            i -= 1
            k = 0  # backspace
            while i >= 0 and not (R[i] != '#' and k == 0):
                if R[i] == '#':
                    k += 1
                else:
                    k -= 1
                i -= 1

            # no more letters left
            return i

        all_strings = [S, T]  # array of strings
        indices = [len(R) for R in all_strings]

        first_string = all_strings[0]

        while True:
            # next indices to compare of all strings
            next_indices = list(map(next_index, all_strings, indices))

            # if we are out of range in all strings - all strings are equal
            if all(j == -1 for j in next_indices):
                return True

            # if we are out of range only in some of the strings - they are not equal
            if any(j == -1 for j in next_indices):
                return False

            # a current letter in the first string
            c = first_string[next_indices[0]]

            # i - a string index, j - a letter index in a string
            for i, j in enumerate(next_indices):
                if c != all_strings[i][j]:
                    return False

            indices = next_indices