A Binary Game

Swayam is inside a maze. The maze can be considered as a binary string which is a combination of 1’s and 0’s. Each door of the maze is either closed which requires a key or is already opened with an attached key that can be used to open any closed door inside the maze. If the doors are the characters of a string ‘S’ then the i-th character of the string denotes the i-th door of the maze where S[i]