面试题：找出最长不重复的子串

题目：给定一个字符串，例如：abbcdef，最长不重复的子串为bcdef。

kotlin代码实现：

fun main() {
    val str = "abbcde"
    var longest = ""
    
    var i = 0
    while (i < str.length) {
        val sb = StringBuffer()
        for (j in i until str.length) {
            val s = str.substring(j, j + 1)
            if (sb.indexOf(s) > 0) {
                i = j - 1
                break
            } else {
                sb.append(s)
            }
        }
        if (sb.length > longest.length) {
            longest = sb.toString()
        }
        i++
    }
    
    println("longest substring:" + longest)
    
}