题目：

题解：

class Solution {
public:int numDistinct(string s, string t) {int m = s.length(), n = t.length();if (m < n) {return 0;}vector<vector<unsigned long long>> dp(m + 1, vector<unsigned long long>(n + 1));for (int i = 0; i <= m; i++) {dp[i][n] = 1;}for (int i = m - 1; i >= 0; i--) {char sChar = s.at(i);for (int j = n - 1; j >= 0; j--) {char tChar = t.at(j);if (sChar == tChar) {dp[i][j] = dp[i + 1][j + 1] + dp[i + 1][j];} else {dp[i][j] = dp[i + 1][j];}}}return dp[0][0];}
};