parse.py

def parse(s):
    """Parse a JSON string containing only integers and arrays."""
    return int(s) if s[0].isdigit() else parse_array(s, 1)[0]

def parse_array(s, i):
    """Return the array beginning at index i and the end index of the array."""
    res = []
    while s[i] != ']':
        if s[i].isdigit():
            start = i
            while s[i].isdigit(): i+= 1
            res.append(int(s[start:i]))
        elif s[i] == '[':
            arr, i = parse_array(s, i+1)
            res.append(arr)
        else: i += 1
    return res, i+1

if __name__ == '__main__':
    tests = [
        1,
        [],
        [1],
        [[]],
        [123],
        [1,2],
        [[1]],
        [[1,2]],
        [[],[]],
        [[[[]]]],
        [1,2,[]],
        [1,2,[3]],
        [[1,2],3],
        [1,[2],3],
        [1,[2,3],4],
        [1,2,[3,4]],
        [1,2,[3,4],5],
        [1,2,[3,4,[5]]],
    ]
    for t in tests:
        assert parse(str(t)) == t
    print 'OK'