1943A - MEX Game 1 - CodeForces Solution

t=int(input())
for j in range(0,t):
    n=int(input())
    a=[0]*n
    string=input()
    b=string.split(' ')
    cnt=[0]*(n+1)
    index=0
    for i in b:
        d=int(i)
        a[index]=d
        index+=1
        cnt[d]+=1
    ans=-1
    f=0
    for i in range(0,n):
        if cnt[i]==0:
            break
        if cnt[i]==1:
            if f==1:
                break
            f=1
            ans=i
        else:
            ans=i
            
    print(ans+1)