#not working def removeDuplicates(self, s: str) -> str: lis=[""] for i in s: if i not in lis: lis.append(i) elif i in lis: lis.remove(i) return ''.join(lis) #working def removeDuplicates(self, s: str) -> str: stack = [""] for c in s: if stack[-1] == c: stack.pop() else: stack.append(c) return ''.join(stack)