LeetCode in Kotlin

3325. Count Substrings With K-Frequency Characters I

Medium

Given a string s and an integer k, return the total number of substrings of s where at least one character appears at least k times.

Example 1:

Input: s = “abacb”, k = 2

Output: 4

Explanation:

The valid substrings are:

• "aba" (character 'a' appears 2 times).
• "abac" (character 'a' appears 2 times).
• "abacb" (character 'a' appears 2 times).
• "bacb" (character 'b' appears 2 times).

Example 2:

Input: s = “abcde”, k = 1

Output: 15

Explanation:

All substrings are valid because every character appears at least once.

Constraints:

• 1 <= s.length <= 3000
• 1 <= k <= s.length
• s consists only of lowercase English letters.

Solution

class Solution {
    fun numberOfSubstrings(s: String, k: Int): Int {
        var left = 0
        var result = 0
        val count = IntArray(26)
        for (i in 0 until s.length) {
            val ch = s[i]
            count[ch.code - 'a'.code]++

            while (count[ch.code - 'a'.code] == k) {
                result += s.length - i
                val atLeft = s[left]
                count[atLeft.code - 'a'.code]--
                left++
            }
        }
        return result
    }
}

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