Distinct Subsequences

DP:

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