def isBalanced(s):
    left, right = ["(", "{", "["], [")", "}", "]"]
    stack = []
    for char in s:
        # add opening brackets to the stack
        if char in left:
            stack.append(char)
        # if it's a right bracket:
        else:       
            # closing bracket appears without an opening bracket
            if not stack:
                return "NO"     
            # check the last opening bracket in the stack
            idx = right.index(char) 
            if stack.pop() == left[idx]:    # remove last bracket in-place
                continue
            return "NO"    # closing bracket didn't find an opening match
    return "NO" if stack else "YES"     # "YES" if stack is empty