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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
int sLen = S.size();
int tLen = T.size();
if(sLen==0 ||tLen == 0)
return 0;
int value[sLen+1][tLen+1];
memset(value,0,sizeof(int)*(sLen+1)*(tLen+1));
for(int i = 0; i< sLen; i++){
if (S[i] == T[0])
value[i+1][1]=value[i][1]+1;
else
value[i+1][1]=value[i][1];
}
for(int i = 0; i< sLen; i++)
for(int j = 1; j<tLen; j++){
if (S[i]==T[j])
value[i+1][j+1] = value[i][j]+value[i][j+1];
else
value[i+1][j+1] = value[i][j+1];
}
return value[sLen][tLen];
}
};
用DPmethod
Value [i][j] = value[i-1][j-1] + value[i-1][j] if (s[i]==s[j])
= value[i-1][j] o.w.
dp式子最开始列错了。写成Value [i][j] = value[i][j-1] o.w., 体会为什么错。(DP式子取决于题目的需求,在本题中,不是求T的substring在S中的情况,而是求整个T的。)
Dp的矩阵到底申请slen还是slen+1取决于第一行/列的赋值情况: 如果第一行/列的取值可以定义用const赋值,就可以申请slen; 如果第一行/列的值需要之前值为参考,则申请+1大小的空间