I am solving a HackerRank problem called 'Morgan and a String'. Given two strings, each consisting of up to 10⁵ uppercase characters, the task is to form the lexicographically smallest string from those letters by taking either of the first remaining character from the strings at each step. Example, for input strings "ACA" and "BCF", the answer should be "ABCACF":

I am solving a HackerRank problem called 'Morgan and a String'. Given two strings, each consisting of up to 10⁵ uppercase characters, the task is to form the lexicographically smallest string from those letters by taking the first remaining character from either of the strings at each step. Example, for input strings "ACA" and "BCF", the answer should be "ABCACF":

I am solving a hackerrank problem called 'Morgan and a String'. I've solved it by writing my own code which works perfectly on smaller test cases.But they run it against test cases which are approx 6000 characters long, So I get a runtime error.
Below is my code:

n=int(input())#number of pairs to input
for mn in range(n):
    a=input()# String1 in the pair
    b=input()#String2 in the pair
    la=[]
    lb=[]

    for i in range(max(len(a),len(b))):#creating lists with all possible elements by slicing off the characters
        la.append(a[i:])
        lb.append(b[i:])

    if len(a)>len(b):#removes empty elements
        lb=[x for x in lb if x!='']
    else:
        la=[x for x in la if x!='']

    while True:#Create empty list for sorting the 0th elements of 'la' nd 'lb'
        temp=[]
        temp.append(la[0])
        temp.append(lb[0])
        temp=sorted(temp)
        print(temp[0][0],end='')#print the 1st character

        if(temp[0] in la):#removing the element after printing the first character
            la.pop(0)
        else:
            lb.pop(0)

        if len(la)==0:#breaks the while loop if a list gets empty
            print(temp[1],end='')
            break
        elif len(lb)==0:
            print(temp[1],end='')
            break

Following are the sample input and output:

ip:1 aceg bdfh

op:abcdefgh

How can I increasing the efficiency?

I am solving a HackerRank problem called 'Morgan and a String'. Given two strings, each consisting of up to 10⁵ uppercase characters, the task is to form the lexicographically smallest string from those letters by taking either of the first remaining character from the strings at each step. Example, for input strings "ACA" and "BCF", the answer should be "ABCACF":

Jack    Daniel  Result
ACA     BCF
CA      BCF     A
CA      CF      AB
A       CF      ABC
A       CF      ABCA
        F       ABCAC
                ABCACF

Sample input, with one test case:

1
ACEG
BDFH

Output:

ABCDEFGH

I've solved it by writing my own code which works perfectly on smaller test cases. But when they run it against test cases which are approx 6000 characters long I get a runtime error. How can I increase the efficiency?

n=int(input())#number of pairs to input
for mn in range(n):
    a=input()# String1 in the pair
    b=input()#String2 in the pair
    la=[]
    lb=[]

    for i in range(max(len(a),len(b))):#creating lists with all possible elements by slicing off the characters
        la.append(a[i:])
        lb.append(b[i:])

    if len(a)>len(b):#removes empty elements
        lb=[x for x in lb if x!='']
    else:
        la=[x for x in la if x!='']

    while True:#Create empty list for sorting the 0th elements of 'la' nd 'lb'
        temp=[]
        temp.append(la[0])
        temp.append(lb[0])
        temp=sorted(temp)
        print(temp[0][0],end='')#print the 1st character

        if(temp[0] in la):#removing the element after printing the first character
            la.pop(0)
        else:
            lb.pop(0)

        if len(la)==0:#breaks the while loop if a list gets empty
            print(temp[1],end='')
            break
        elif len(lb)==0:
            print(temp[1],end='')
            break