1944C - MEX Game 1 - CodeForces Solution

t = int(input())
while t > 0:
    n = int(input())
    arr = list(map(int, input().split()))
    
    x = 0
    from collections import Counter
    c = Counter(arr)
    for i in range(0, n+1):
        if c.get(i, 0) == 0:
            print(i)
            break
        if c.get(i) == 1:
            x += 1
        if x == 2:
            print(i)
            break
    t -= 1