Given a string S and a string T, count the number of distinct subsequences of S which equals T.
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).
Example 1:
Input: S ="rabbbit", T ="rabbit" Output: 3Explanation: As shown below, there are 3 ways you can generate "rabbit" from S. (The caret symbol ^ means the chosen letters)rabbbit^^^^ ^^rabbbit^^ ^^^^rabbbit^^^ ^^^
Example 2:
Input: S ="babgbag", T ="bag" Output: 5Explanation: As shown below, there are 5 ways you can generate "bag" from S. (The caret symbol ^ means the chosen letters)babgbag^^ ^babgbag^^ ^babgbag^ ^^babgbag^ ^^babgbag^^^
分析：
这题就不装了，没想出来，看的牛客网的解析，很厉害，怎么想到的....想了半天没想懂二维的dp表怎么建。
https://www.nowcoder.com/questionTerminal/ed2923e49d3d495f8321aa46ade9f873
代码：
class Solution {
public:
int numDistinct(string s, string t) {
if(s.empty())
return 0;
if(t.empty())
return 1;
vector<int> init(s.size()+1,0);
vector<vector<int>> table(t.size()+1, init);
for(int i=0;i<=s.size();++i)
table[0][i]=1;
for(int i=1;i<=t.size();++i)
{
for(int j=1;j<=s.size();++j)
{
if(s[j-1]==t[i-1])
{
table[i][j]=table[i][j-1]+table[i-1][j-1];
}
else
{
table[i][j]=table[i][j-1];
}
}
}
return table[t.size()][s.size()];
}
};