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Distinct Subsequences

Given a string S and a string T, count the number of distinct subsequences of T in S.

A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, "ACE" is a subsequence of "ABCDE" while "AEC" is not).

Here is an example:
S = "rabbbit", T = "rabbit"

Return 3.

class Solution {public:    int numDistinct(string S, string T) {        int m = T.length();        int n = S.length();        if (m > n) return 0;            vector<vector<int>> path(m+1, vector<int>(n+1, 0));        for (int k = 0; k <= n; k++) path[0][k] = 1;                for (int j = 1; j <= n; j++) {            for (int i = 1; i <= m; i++) {                path[i][j] = path[i][j-1] + (T[i-1] == S[j-1] ? path[i-1][j-1] : 0);            }        }            return path[m][n];    }};

 

Distinct Subsequences