How to Solve the Longest Substring Problem

Find the longest sub-string of the given string that contains, at most, two unique characters. If you find multiple sub-strings that match the description, print the last sub-string (furthest to the right).

Inputs:

abbccc
abcabcabcabccc
qwertyytrewq
aaabaaabccbcccbde
abcde
aaaa
aabb

Outputs:

bbccc
bccc
tyyt
bccbcccb
de
aaaa
aabb

?

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package main   import (     "fmt" )   type Substring struct {     begin int     end   int }   func longestSubstring(str string) string {     m := map[uint8]bool{}     substrings := []Substring{}     for i := 0; i < len(str); i++ {         if _, ok := m[str[i]]; !ok {             m[str[i]] = true         }         if len(m) == 3 {             m = map[uint8]bool{}             m[str[i-1]] = true             m[str[i]] = true             if len(substrings) == 0 {                 substrings = append(substrings, Substring{0, i - 1})             } else {                 begin := substrings[len(substrings)-1].end                 c := str[begin]                 for j := begin; str[j] == c && j != 0; j-- {                     begin--                 }                 substrings = append(substrings, Substring{begin + 1, i - 1})             }         }     }     if len(m) == 2 {         if len(substrings) == 0 {             substrings = append(substrings, Substring{0, len(str) - 1})         } else {             begin := substrings[len(substrings)-1].end             c := str[begin]             for j := begin; str[j] == c && j != 0; j-- {                 begin--             }             substrings = append(substrings, Substring{begin + 1, len(str) - 1})         }     } else {         return str     }     var longest string     for _, v := range substrings {         substring := str[v.begin : v.end+1]         if len(longest) <= len(substring) {             longest = substring         }     }     return longest }   func main() {     fmt.Println(longestSubstring("abbccc"))     fmt.Println(longestSubstring("abcabcabcabccc"))     fmt.Println(longestSubstring("qwertyytrewq"))     fmt.Println(longestSubstring("aaabaaabccbcccbde"))     fmt.Println(longestSubstring("abcde"))     fmt.Println(longestSubstring("aaaa"))     fmt.Println(longestSubstring("aabb")) }