1170. Shortest Common Supersequence
My accepted Python solution to LeetCode problem 1170, Shortest Common Supersequence, running in 406ms.
- Difficulty: Hard
- Python
- Runtime 406ms
- Memory 42.3MB
- Updated
Read the problem on LeetCode View on GitHub
The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.
Python
Accepted on LeetCode — runtime 406ms, memory 42.3MB, accepted 2026-01-02.
class Solution:
def shortestCommonSupersequence(self, str1: str, str2: str) -> str:
m, n = len(str1), len(str2)
# DP for LCS
dp = [[0] * (n + 1) for _ in range(m + 1)]
for i in range(1, m + 1):
for j in range(1, n + 1):
if str1[i - 1] == str2[j - 1]:
dp[i][j] = dp[i - 1][j - 1] + 1
else:
dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
# Backtrack to build shortest common supersequence
result = []
i, j = m, n
while i > 0 and j > 0:
if str1[i - 1] == str2[j - 1]:
result.append(str1[i - 1])
i -= 1
j -= 1
elif dp[i - 1][j] > dp[i][j - 1]:
result.append(str1[i - 1])
i -= 1
else:
result.append(str2[j - 1])
j -= 1
while i > 0:
result.append(str1[i - 1])
i -= 1
while j > 0:
result.append(str2[j - 1])
j -= 1
return ''.join(reversed(result))