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Program to find number of distinct subsequences in Python
Suppose we have a string s, we have to count the number of distinct subsequences of the string s. If the answer is too large then return result modulo 10^9 + 7.
So, if the input is like s = "bab", then the output will be 6 because there are 6 different sequences, these are "a", "b, "ba", "ab", "bb", "abb".
To solve this, we will follow these steps −
dp := an array whose size is same of s and filled with 0
m := 10^9 + 7
for each index i and item char in s, do
ind := index of i-th char in s from right
dp[i] := 1 + (sum of all elements in dp[from index 0 to i-1]) mod m if ind is same as -1 otherwise (sum of all elements in dp[from index ind to i-1]) mod m
return sum of all elements in dp mod m
Example
Let us see the following implementation to get better understanding
def solve(s):
dp, m = [0] * len(s), 10**9 + 7
for i, char in enumerate(s):
ind = s.rfind(char, 0, i)
dp[i] = 1 + sum(dp[:i]) % m if ind == -1 else sum(dp[ind:i]) % m
return sum(dp) % m
s = "bab"
print(solve(s))Input
"abcd", "abcdbcd"
Output
6
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