一、题目

Given two strings s and t, return the number of distinct subsequences of s which equals t.

The test cases are generated so that the answer fits on a 32-bit signed integer.

Example 1:

Input: s = “rabbbit”, t = “rabbit”
Output: 3
Explanation:
As shown below, there are 3 ways you can generate “rabbit” from s.
rabbbit
rabbbit
rabbbit
Example 2:

Input: s = “babgbag”, t = “bag”
Output: 5
Explanation:
As shown below, there are 5 ways you can generate “bag” from s.
babgbag
babgbag
babgbag
babgbag
babgbag

Constraints:

1 <= s.length, t.length <= 1000
s and t consist of English letters.

二、题解

class Solution {
public:int numDistinct(string s, string t) {int mod = 1e9+7;int n = s.size(),m = t.size();vector<vector<int>> dp(n+1,vector<int>(m+1,0));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++){dp[i][j] = dp[i-1][j];if(s[i-1] == t[j-1]) dp[i][j] += dp[i-1][j-1];dp[i][j] = dp[i][j] % mod;}}return dp[n][m] % mod;}
};