Leetcode #115: Distinct Subsequences

First try failed at the test case
["fff", "ffff"] and ["avc", ""]

func numDistinct(_ s: String, _ t: String) -> Int {
    if t.count == 0 {
        return 1
    }
    if s.count < t.count {
        return 0
    }
    let S = Array(s)
    let T = Array(t)
    var tmp = Array(repeating: 1, count: S.count + 1)
    var dp = [Int]()
    for i in 1...T.count {
        dp = Array(repeating: 0, count: S.count + 1)
        for j in i...S.count {
            if S[j-1] == T[i-1] {
                dp[j] = dp[j-1] + tmp[j-1]
            } else {
                dp[j] = dp[j-1]
            }
        }
        tmp = dp
    }
    return dp[S.count]
}