Given two strings s and t, return the number of distinct subsequences of s which equals t.
A string's subsequence is a new string formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., "ACE" is a subsequence of "ABCDE" while "AEC" is not).
It's guaranteed the answer fits on a 32-bit signed integer.
Example 1:
Input: s = "rabbbit", t = "rabbit"
Output: 3
Explanation:
As shown below, there are 3 ways you can generate "rabbit" from S.
rabbbit
rabbbit
rabbbit
Example 2:
Input: s = "babgbag", t = "bag"
Output: 5
Explanation:
As shown below, there are 5 ways you can generate "bag" from S.
babgbag
babgbag
babgbag
babgbag
babgbag
class Solution:
def numDistinct(self, s: str, t: str) -> int:
m, n = len(s), len(t)
dp = [[0] * (m+1) for _ in range(n+1)]
for i in range(m+1):
dp[0][i] = 1
for i in range(1, n+1):
for j in range(1, m+1):
if s[j-1] == t[i-1]:
dp[i][j] = dp[i][j-1] + dp[i-1][j-1]
else:
dp[i][j] = dp[i][j-1]
return dp[n][m]