def main():
    length = int(input())
    st = input()

    counts = [(0, 0) for i in range(len(st)+1)]

    found = set()
    cur = 0
    for i in range(len(st)):
        c = st[i]
        if c not in found:
            found.add(c)
            cur += 1
        counts[i+1] = (cur, counts[i][1])

    found = set()
    cur = 0
    for i in range(len(st)-1, -1, -1):
        c = st[i]
        if c not in found:
            found.add(c)
            cur += 1
        counts[i] = (counts[i][0], cur)

    # print(counts)
    return max([(c[0]+c[1]) for c in counts])


for _ in range(int(input())):
    print(main())