LeetCode Entry

1358. Number of Substrings Containing All Three Characters

30.06.2026 medium 2026 kotlin rust

Substrings with 3 letters

1358. Number of Substrings Containing All Three Characters medium substack youtube

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Problem TLDR

Substrings with 3 letters

Intuition

Sum of substrings ending at position i. Count all prefixes.

Approach

• just remember the latest position of each letter, the min would be the prefix

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(1)\)

Code

    fun numberOfSubstrings(s: String) = IntArray(3).run {
        s.indices.sumOf { i -> set(s[i]-'a', i+1); min() }
    }

    pub fn number_of_substrings(s: String) -> i32 {
        s.bytes().zip(1..).fold(([0;3],0),|(mut j,mut s),(b,i)|{
            j[b as usize%3]=i;(j,s+j[0].min(j[1]).min(j[2]))}).1
    }