HackerRank "Morgan and a String"

I saw the same sub-problem in LeetCode, and there exists a O(n) neat greedy solution:

for _ in range(int(input())):
    a = input() + '['
    b = input() + '['

    output = ""
    for _ in range(len(a) + len(b) - 2):
        if a < b:
            output += a[0]
            a = a[1:]
        else:
            output += b[0]
            b = b[1:]

    print(output)

Please note: '[' is the first char after 'Z'.