solution.py

# Problem 1
def reverse_integer(n):
    LOWER = -(2**31)
    UPPER = (2**31)-1
    n_str = str(n)
    n_digit_list = list(n_str)
    n_digit_list.reverse()
    reverse_str = ''.join(n_digit_list)

    if reverse_str.endswith('-'):
        result = -1 * int(reverse_str[:-1])
    else:
        result = int(reverse_str)

    if result < LOWER or result > UPPER:
        return 0
    
    return result

# Problem 2
def is_valid(input):
    closing_map = {
        '(': ')',
        '{': '}',
        '[': ']'
    }

    stack = []

    for current in input:
        if current in '({[':
            stack.append(current)
        else:
            if len(stack) == 0:
                return False
            last = stack.pop()

            if closing_map[last] != current:
                return False
            
    return True

# Problem 3
def max_sub_array(array):
    max = -1e20

    for i in range(0, len(array)+1):
        for j in range(i, len(array)+1):
            s = sum(array[i:j])
            if s > max:
                max = s
    
    return max