Buggy python submission (verdict: Time Limit Exceeded):
#!usr/bin/env python3
from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
from functools import reduce
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LS()
    return l
_gcd = lambda x, y: _gcd(y, x%y) if (x%y) else y
_lcm = lambda x, y: x*y // _gcd(x, y)
def gcd(*numbers):
    return reduce(_gcd, numbers)
def lcm(*numbers):
    return reduce(_lcm, numbers)
def factorial(n):
    if (n==0) or (n==1):
        return 1
    ret = 1
    for i in range(2, n+1):
        ret *= i
    return ret
def combination(n, k):
    return factorial(n)//factorial(k)//factorial(n-k)
sys.setrecursionlimit(1000000)
INF = float('inf')
mod = 1000000007
dire4 = [(1,0), (0,1), (-1,0), (0,-1)]
dire8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)]

s = S()
t = S()

#dp[s][t]
dp = [[0]*(len(t)+1) for i in range(len(s)+1)]

for i in range(len(s)):
    for j in range(len(t)):
        if (s[i] == t[j]):
            dp[i+1][j+1] = dp[i][j] + 1
        else:
            dp[i+1][j+1] = max(dp[i+1][j], dp[i][j+1])

i = len(s)
j = len(t)
res = ""
while (i>0  and j>0):
    if (dp[i][j] == dp[i-1][j]):
        i -= 1
    elif (dp[i][j] == dp[i][j-1]):
        j -= 1
    else:
        i -= 1
        j -= 1
        res = s[i] + res

print(res)

Fixed submission:
#!usr/bin/env python3
from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
from functools import reduce
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LS()
    return l
_gcd = lambda x, y: _gcd(y, x%y) if (x%y) else y
_lcm = lambda x, y: x*y // _gcd(x, y)
def gcd(*numbers):
    return reduce(_gcd, numbers)
def lcm(*numbers):
    return reduce(_lcm, numbers)
def factorial(n):
    if (n==0) or (n==1):
        return 1
    ret = 1
    for i in range(2, n+1):
        ret *= i
    return ret
def combination(n, k):
    return factorial(n)//factorial(k)//factorial(n-k)
sys.setrecursionlimit(1000000)
INF = float('inf')
mod = 1000000007
dire4 = [(1,0), (0,1), (-1,0), (0,-1)]
dire8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)]

s = S()
t = S()

#dp[s][t]
dp = [[0]*(len(t)+1) for i in range(len(s)+1)]

for i in range(len(s)):
    for j in range(len(t)):
        if (s[i] == t[j]):
            dp[i+1][j+1] = dp[i][j] + 1
        else:
            dp[i+1][j+1] = max(dp[i+1][j], dp[i][j+1])

i = len(s)
j = len(t)
res = ""
while (i>0  and j>0):
    if (dp[i][j] == dp[i-1][j]):
        i -= 1
    elif (dp[i][j] == dp[i][j-1]):
        j -= 1
    else:
        i -= 1
        j -= 1
        res = s[i] + res

print(res)

Buggy python submission (verdict: Time Limit Exceeded):
#!usr/bin/env python3
from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
from functools import reduce
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LS()
    return l
_gcd = lambda x, y: _gcd(y, x%y) if (x%y) else y
_lcm = lambda x, y: x*y // _gcd(x, y)
def gcd(*numbers):
    return reduce(_gcd, numbers)
def lcm(*numbers):
    return reduce(_lcm, numbers)
def factorial(n):
    if (n==0) or (n==1):
        return 1
    ret = 1
    for i in range(2, n+1):
        ret *= i
    return ret
def combination(n, k):
    return factorial(n)//factorial(k)//factorial(n-k)
sys.setrecursionlimit(1000000)
INF = float('inf')
mod = 1000000007
dire4 = [(1,0), (0,1), (-1,0), (0,-1)]
dire8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)]

s = S()
t = S()

#dp[s][t]
dp = [[""]*(len(t)+1) for i in range(len(s)+1)]

for i in range(len(s)):
    for j in range(len(t)):
        if (s[i] == t[j]):
            dp[i+1][j+1] = dp[i][j] + s[i]
        else:
            dp[i+1][j+1] = dp[i+1][j] if len(dp[i+1][j])>=len(dp[i][j+1]) else dp[i][j+1]

print(dp[len(s)][len(t)])

Fixed submission:
#!usr/bin/env python3
from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
from functools import reduce
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LS()
    return l
_gcd = lambda x, y: _gcd(y, x%y) if (x%y) else y
_lcm = lambda x, y: x*y // _gcd(x, y)
def gcd(*numbers):
    return reduce(_gcd, numbers)
def lcm(*numbers):
    return reduce(_lcm, numbers)
def factorial(n):
    if (n==0) or (n==1):
        return 1
    ret = 1
    for i in range(2, n+1):
        ret *= i
    return ret
def combination(n, k):
    return factorial(n)//factorial(k)//factorial(n-k)
sys.setrecursionlimit(1000000)
INF = float('inf')
mod = 1000000007
dire4 = [(1,0), (0,1), (-1,0), (0,-1)]
dire8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)]

s = S()
t = S()

#dp[s][t]
dp = [[0]*(len(t)+1) for i in range(len(s)+1)]

for i in range(len(s)):
    for j in range(len(t)):
        if (s[i] == t[j]):
            dp[i+1][j+1] = dp[i][j] + 1
        else:
            dp[i+1][j+1] = max(dp[i+1][j], dp[i][j+1])

i = len(s)
j = len(t)
res = ""
while (i>0  and j>0):
    if (dp[i][j] == dp[i-1][j]):
        i -= 1
    elif (dp[i][j] == dp[i][j-1]):
        j -= 1
    else:
        i -= 1
        j -= 1
        res = s[i] + res

print(res)

Buggy python submission (verdict: Time Limit Exceeded):
#!usr/bin/env python3
from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
from functools import reduce
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LS()
    return l
_gcd = lambda x, y: _gcd(y, x%y) if (x%y) else y
_lcm = lambda x, y: x*y // _gcd(x, y)
def gcd(*numbers):
    return reduce(_gcd, numbers)
def lcm(*numbers):
    return reduce(_lcm, numbers)
def factorial(n):
    if (n==0) or (n==1):
        return 1
    ret = 1
    for i in range(2, n+1):
        ret *= i
    return ret
def combination(n, k):
    return factorial(n)//factorial(k)//factorial(n-k)
sys.setrecursionlimit(1000000)
INF = float('inf')
mod = 1000000007
dire4 = [(1,0), (0,1), (-1,0), (0,-1)]
dire8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)]

s = S()
t = S()

#dp[s][t]
dp = [[""]*(len(t)+1) for i in range(len(s)+1)]

for i in range(len(s)):
    for j in range(len(t)):
        if (s[i] == t[j]):
            dp[i+1][j+1] = dp[i][j] + s[i]
        else:
            dp[i+1][j+1] = dp[i+1][j] if len(dp[i+1][j])>=len(dp[i][j+1]) else dp[i][j+1]

print(dp[len(s)][len(t)])

Fixed submission:
#!usr/bin/env python3
from collections import defaultdict
from collections import deque
from heapq import heappush, heappop
from functools import reduce
import sys
import math
import bisect
import random
def LI(): return list(map(int, sys.stdin.readline().split()))
def I(): return int(sys.stdin.readline())
def LS():return list(map(list, sys.stdin.readline().split()))
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = I()
    return l
def LIR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LI()
    return l
def SR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = S()
    return l
def LSR(n):
    l = [None for i in range(n)]
    for i in range(n):l[i] = LS()
    return l
_gcd = lambda x, y: _gcd(y, x%y) if (x%y) else y
_lcm = lambda x, y: x*y // _gcd(x, y)
def gcd(*numbers):
    return reduce(_gcd, numbers)
def lcm(*numbers):
    return reduce(_lcm, numbers)
def factorial(n):
    if (n==0) or (n==1):
        return 1
    ret = 1
    for i in range(2, n+1):
        ret *= i
    return ret
def combination(n, k):
    return factorial(n)//factorial(k)//factorial(n-k)
sys.setrecursionlimit(1000000)
INF = float('inf')
mod = 1000000007
dire4 = [(1,0), (0,1), (-1,0), (0,-1)]
dire8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)]

s = S()
t = S()

#dp[s][t]
dp = [[0]*(len(t)+1) for i in range(len(s)+1)]

for i in range(len(s)):
    for j in range(len(t)):
        if (s[i] == t[j]):
            dp[i+1][j+1] = dp[i][j] + 1
        else:
            dp[i+1][j+1] = max(dp[i+1][j], dp[i][j+1])

i = len(s)
j = len(t)
res = ""
while (i>0  and j>0):
    if (dp[i][j] == dp[i-1][j]):
        i -= 1
    elif (dp[i][j] == dp[i][j-1]):
        j -= 1
    else:
        i -= 1
        j -= 1
        res = s[i] + res

print(res)