package main

import "sync/atomic"

func longestDupSubstring(s string) string {
	n := len(s)
	for l := n - 1; l > 0; l-- {
		book := make(map[string]bool)
		for i := 0; i+l < n; i++ {
			str := s[i : i+l]
			if book[str] {
				return str
			}
			book[s] = true
		}
	}
	atomic.CompareAndSwapUint32()
	return ""
}

func main() {

}