Buggy python submission (verdict: Time Limit Exceeded):
s = input()
t = input()
ls = len(s)
lt = len(t)
DP_len = [[0] * (lt+1) for i in range(ls+1)]
DP = [[''] * (lt+1) for i in range(ls+1)]
for i in range(1, ls+1):
    for j in range(1, lt+1):
        if s[i-1] == t[j-1]:
            if (DP_len[i-1][j-1] + 1 >= DP_len[i][j-1]) and (DP_len[i-1][j-1] + 1 >= DP_len[i-1][j]):
                DP[i][j] = DP[i-1][j-1] + s[i-1]
                DP_len[i][j] = DP_len[i - 1][j - 1] + 1
            else:
                if (DP_len[i-1][j] >= DP_len[i][j-1]) and (DP_len[i-1][j] >= DP_lenDP[i][j]):
                    DP[i][j] = DP[i-1][j]
                    DP_len[i][j] = DP_len[i - 1][j]
                elif (DP_len[i-1][j] <= DP_len[i][j-1]) and (DP_len[i][j-1] >= DP_len[i][j]):
                    DP[i][j] = DP[i][j-1]
                    DP_len[i][j] = DP_len[i][j-1]
        else:
            if (DP_len[i - 1][j] >= DP_len[i][j - 1]) and (DP_len[i - 1][j] >= DP_len[i][j]):
                DP[i][j] = DP[i - 1][j]
                DP_len[i][j] = DP_len[i - 1][j]
            elif (DP_len[i - 1][j] <= DP_len[i][j - 1]) and (DP_len[i][j - 1] >= DP_len[i][j]):
                DP[i][j] = DP[i][j - 1]
                DP_len[i][j] = DP_len[i][j - 1]

print(DP[ls][lt])



Fixed submission:
from collections import deque

s = input()
t = input()
ls = len(s)
lt = len(t)
DP_len = [[0] * (lt + 1) for i in range(ls + 1)]
Prev =[[0] * (lt + 1) for i in range(ls + 1)]
for i in range(1, ls + 1):
    for j in range(1, lt + 1):
        if s[i - 1] == t[j - 1]:
            if max(DP_len[i - 1][j - 1] + 1, DP_len[i][j - 1], DP_len[i - 1][j]) == DP_len[i-1][j-1] + 1:
                DP_len[i][j] = DP_len[i-1][j-1] + 1
                Prev[i][j] = (i-1, j-1)
            else:
                if DP_len[i][j - 1] >= DP_len[i - 1][j]:
                    DP_len[i][j] = DP_len[i][j - 1]
                    Prev[i][j] = (i, j - 1)
                else:
                    DP_len[i][j] = DP_len[i - 1][j]
                    Prev[i][j] = (i - 1, j)

        else:
            if DP_len[i][j - 1] >= DP_len[i - 1][j]:
                DP_len[i][j] = DP_len[i][j - 1]
                Prev[i][j] = (i, j-1)
            else:
                DP_len[i][j] = DP_len[i - 1][j]
                Prev[i][j] = (i - 1, j)

x = ls
y = lt
ans = ''
while True:
    if x == 0 or y == 0:
        break
    nx, ny = Prev[x][y]
    if (nx != x) and (ny != y):
        ans = s[x-1] + ans
    x = nx
    y = ny
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
s = input()
t = input()
ls = len(s)
lt = len(t)
DP = [[''] * (lt+1) for i in range(ls+1)]
for i in range(1, ls+1):
    for j in range(1, lt+1):
        if s[i-1] == t[j-1]:
            if (len(DP[i-1][j-1]) + 1 >= len(DP[i][j-1])) and (len(DP[i-1][j-1]) + 1 >= len(DP[i-1][j])):
                DP[i][j] = DP[i-1][j-1] + s[i-1]
                flag = False
            else:
                if (len(DP[i-1][j]) >= len(DP[i][j-1])) and (len(DP[i-1][j]) >= len(DP[i][j])):
                    DP[i][j] = DP[i-1][j]
                elif (len(DP[i-1][j]) <= len(DP[i][j-1])) and (len(DP[i][j-1]) >= len(DP[i][j])):
                    DP[i][j] = DP[i][j-1]

        else:
            if (len(DP[i - 1][j]) >= len(DP[i][j - 1])) and (len(DP[i - 1][j]) >= len(DP[i][j])):
                DP[i][j] = DP[i - 1][j]
            elif (len(DP[i - 1][j]) <= len(DP[i][j - 1])) and (len(DP[i][j - 1]) >= len(DP[i][j])):
                DP[i][j] = DP[i][j - 1]

print(DP[ls][lt])



Fixed submission:
from collections import deque

s = input()
t = input()
ls = len(s)
lt = len(t)
DP_len = [[0] * (lt + 1) for i in range(ls + 1)]
Prev =[[0] * (lt + 1) for i in range(ls + 1)]
for i in range(1, ls + 1):
    for j in range(1, lt + 1):
        if s[i - 1] == t[j - 1]:
            if max(DP_len[i - 1][j - 1] + 1, DP_len[i][j - 1], DP_len[i - 1][j]) == DP_len[i-1][j-1] + 1:
                DP_len[i][j] = DP_len[i-1][j-1] + 1
                Prev[i][j] = (i-1, j-1)
            else:
                if DP_len[i][j - 1] >= DP_len[i - 1][j]:
                    DP_len[i][j] = DP_len[i][j - 1]
                    Prev[i][j] = (i, j - 1)
                else:
                    DP_len[i][j] = DP_len[i - 1][j]
                    Prev[i][j] = (i - 1, j)

        else:
            if DP_len[i][j - 1] >= DP_len[i - 1][j]:
                DP_len[i][j] = DP_len[i][j - 1]
                Prev[i][j] = (i, j-1)
            else:
                DP_len[i][j] = DP_len[i - 1][j]
                Prev[i][j] = (i - 1, j)

x = ls
y = lt
ans = ''
while True:
    if x == 0 or y == 0:
        break
    nx, ny = Prev[x][y]
    if (nx != x) and (ny != y):
        ans = s[x-1] + ans
    x = nx
    y = ny
print(ans)