Given a string of characters, find the number of index pairs i and j (i <= j) between which there are no repeating characters, inclusive. For the string "aba", the answer is 5: [0, 0] ("a") [0, 1] ("ab") [1, 1] ("b") [1, 2] ("ba") [2, 2] ("a") For the string "acbb", what is the answer?

For the string "acbb", you need to find the number of index pairs [i, j] (i ≤ j) such that the substring s[i..j] contains only unique characters.

Let's analyze the string "acbb":

• [0,0] = "a" — unique
• [0,1] = "ac" — unique
• [0,2] = "acb" — unique
• [0,3] = "acbb" — repeated "b", not suitable
• [1,1] = "c" — unique
• [1,2] = "cb" — unique
• [1,3] = "cbb" — repeated "b", not suitable
• [2,2] = "b" — unique
• [2,3] = "bb" — repeated "b", not suitable
• [3,3] = "b" — unique

Total of 7 such pairs.

This problem can be solved using two pointers and a set to track unique characters:

func countUniqueSubstrings(s string) int {
    seen := make(map[rune]bool)
    count := 0
    left := 0
    for right, ch := range s {
        for seen[ch] {
            delete(seen, rune(s[left]))
            left++
        }
        seen[ch] = true
        count += right - left + 1
    }
    return count
}

// Example usage:
// fmt.Println(countUniqueSubstrings("acbb")) // Outputs 7

This algorithm runs in O(n), where n is the length of the string.