828. Count Unique Characters of All Substrings of a Given String

📋 Đề Bài

Let's define a function countUniqueChars(s) that returns the number of unique characters in s.

For example, calling countUniqueChars(s) if s = "LEETCODE" then "L", "T", "C", "O", "D" are the unique characters since they appear only once in s, therefore countUniqueChars(s) = 5.

Given a string s, return the sum of countUniqueChars(t) where t is a substring of s. The test cases are generated such that the answer fits in a 32-bit integer.

Notice that some substrings can be repeated so in this case you have to count the repeated ones too.

Example 1:

Input: s = "ABC"
Output: 10
Explanation: All possible substrings are: "A","B","C","AB","BC" and "ABC".
Every substring is composed with only unique letters.
Sum of lengths of all substring is 1 + 1 + 1 + 2 + 2 + 3 = 10

Example 2:

Input: s = "ABA"
Output: 8
Explanation: The same as example 1, except countUniqueChars("ABA") = 1.

Example 3:

Input: s = "LEETCODE"
Output: 92

Constraints:

1 <= s.length <= 10⁵
s consists of uppercase English letters only.

🧠 Thuật Toán & Kỹ Thuật

String (Chuỗi)

⏱️ Thời gian O(n²)

💾 Không gian O(1)

💻 Lời Giải

C++ 0828-count-unique-characters-of-all-substrings-of-a-given-string.cpp

class Solution {
public:
    int uniqueLetterString(string s) {
        const int n = s.size();
        int ans = 0;
        const int MOD = 1e9 + 7;
        
        for (int i = 0, l = i, r = i; i < n; ++i) {
            for (l = i - 1; l >= 0 && s[i] != s[l]; --l);
            for (r = i + 1; r < n && s[i] != s[r]; ++r);
            ans += ((i - l)*(r - i)) % MOD;
        }
        
        return ans;
    }
};