题目：

题解：

func numDistinct(s, t string) int {
    m, n := len(s), len(t)
    if m < n {
        return 0
    }
    dp := make([][]int, m+1)
    for i := range dp {
        dp[i] = make([]int, n+1)
        dp[i][n] = 1
    }
    for i := m - 1; i >= 0; i-- {
        for j := n - 1; j >= 0; j-- {
            if s[i] == t[j] {
                dp[i][j] = dp[i+1][j+1] + dp[i+1][j]
            } else {
                dp[i][j] = dp[i+1][j]
            }
        }
    }
    return dp[0][0]
}