题目：

题解：

int numDistinct(char* s, char* t) {
    int m = strlen(s), n = strlen(t);
    if (m < n) {
        return 0;
    }
    unsigned long long dp[m + 1][n + 1];
    memset(dp, 0, sizeof(dp));
    for (int i = 0; i <= m; i++) {
        dp[i][n] = 1;
    }
    for (int i = m - 1; i >= 0; i--) {
        char sChar = s[i];
        for (int j = n - 1; j >= 0; j--) {
            char tChar = t[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];
}