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;}int[][] dp = new int[m + 1][n + 1];for (int i = 0; i <= m; i++) {dp[i][n] = 1;}for (int i = m - 1; i >= 0; i--) {char sChar = s.charAt(i);for (int j = n - 1; j >= 0; j--) {char tChar = t.charAt(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];}
}