算法字符串匹配

给你⼀个仅包含⼩写字⺟的字符串主串S = "abcacabdc"，模式串T = "abd"，请查找出模式串在主串第⼀次出现的位置

提示：主串和模式串均为⼩写字⺟且都是合法输⼊

S = "abcacabdc"
T = "abc"
返回 1
S = "abcacabdc"
T = "abd"
返回 6

    func thread(S:String,T:String) -> Int {
        //abcacabdc   //abd
        var index = 0
        var i = 0
        for h in S {
            i = index
            index+=1
            var j = 1
            for c2 in T {
                j+=1
                let position = S.index(S.startIndex, offsetBy: i) //获取String.index
                let c1 = S[position]
//                let c1 = S[i]
                print("外字符串字符：\(c1)")
                print("内部字符串字符：\(c2)")
                if c1 != c2 {
                    print("++++第\(index)次++++")
                    break
                }else{
                    if j == T.count {
                        return  index
                    }
                }
                i+=1
            }
        }
        return -1
    }