Leetcode Distinct subsequences-----java

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 was formed from the original string by deleting some (can be none) of the C Haracters without disturbing the relative positions of the remaining characters. (ie, is a subsequence of and is not "ACE" "ABCDE" "AEC" ).

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

Return 3 .

The problem is that the string T is the number of all possibilities of string s strings (no existence is 0).

At the beginning of the calculation is more violent, so timed out.

 Public classSolution {Char[] word1; Char[] word2; intresult = 0;  Public intnumdistinct (string s, String t) {intLen1 = S.length (), len2 =t.length (); if(Len1 <len2)return0; Word1=S.tochararray (); Word2=T.tochararray ();  for(inti = 0;i<=len1-len2;i++){            if(Word1[i] = = Word2[0]) Helper (i+1,1); }        returnresult; }     Public voidHelperintStart1,intStart2) {                if(Start2 = =word2.length) {Result++; return ; }         for(inti = start1;i< word1.length; i++){            if(Word1[i] = =Word2[start2]) helper (i+1,start2+1); }                            }}

So use the DP algorithm.

DP, the method of normalization of two-dimensional map.

R a b b i t

1 0 0 0 0 0 0

R 1

A 1

B 1

B 1

B 1

I 1

T 1

If the current character is the same, dp[i][j] result equals (Dp[i-1][j-1]) and (Dp[i-1][j]) sum

If the current character is different, dp[i][j] = Dp[i-1][j]

 Public classSolution { Public intnumdistinct (string s, String t) {intLen1 = S.length (), len2 =t.length (); if(Len1 <len2)return0; Char[] Word1 =S.tochararray (); Char[] Word2 =T.tochararray (); int[] DP =New int[Len1+1] [Len2+1];  for(inti = 0;i<=len1;i++){             for(intj = 0;j<=i && j<=len2;j++){                        if(i = = 0 && J! = 0) Dp[i][j]= 0; Else if(j = = 0) Dp[i][j]= 1; Else if(Word1[i-1] = = Word2[j-1]) Dp[i][j]= Dp[i-1][j-1] + dp[i-1][j]; ElseDp[i][j]= Dp[i-1][j]; }        }            returnDp[len1][len2]; }            }

Leetcode Distinct subsequences-----java