I was solving a simple DP problem ..
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.
I made a recursive code which is like this ..
I initialise the DP array with -1 and used one based indexing in DP matrix ..
int solve(string S,string T,int N,int M){if( M <= 0) return 1 ; if( N <= 0 ) return 0 ; int &ret = DP[N][M]; if(ret != -1) return ret ; ret = 0 ; if(S[N-1] == T[M-1]){ ret += (solve(S,T,N-1,M-1)+solve(S,T,N-1,M)) ; }else{ ret += (solve(S,T,N-1,M)) ; } ret ;}
I was not getting correct answer then i changed my code and prepared a bottom up code which is this ..
int solve(string &S,string &T,int N,int M){ for(int i=0;i<=N;i++) for(int j=0;j<=M;j++) DP[i][j] = 0; for(int i=0;i<=N;i++) DP[i][0] = 1; for(int i=1;i<=N;i++){ for(int j=1;j<=M;j++){ if(S[i-1] == T[j-1]) DP[i][j] += DP[i-1][j-1] ; DP[i][j] += DP[i-1][j] ; } } return DP[N][M] ; }
I got AC with this solution ..Essentially the same as previous one ..
I am still unable to find a bug in my previous code so can anyone help me please ..
Thanx in advance
What's with the last line of the solve function? I guess it should be "return ret;". Add -Wreturn-type (or -Wall if you want many other useful checks) to your compiler options and it shouldn't happen again.
Yes you were right. Thanx dude