LeetCode Entry

2138. Divide a String Into Groups of Size k

22.06.2025 easy 2025 kotlin rust

k-chunked string, filled tail

2138. Divide a String Into Groups of Size k easy blog post substack youtube

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Problem TLDR

k-chunked string, filled tail #easy

Intuition

Pad then chunk, or chunk then pad. Prefill everything, or write a precise filling code. Great task to learn the language built-ins.

Approach

• if you know Kotlin padEnd & chunked you don’t have to think
• Rust doesn’t allow fmt with dynamic fill character
• 1 + (size - 1) / k or (k + size - 1) / k will pad to % k
• Kotln / Java String has CharArray constructor arguments
• Rust has resize to pad-fill a Vec

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(n)\)

Code

// 14ms
    fun divideString(s: String, k: Int, fill: Char) =
        s.chunked(k).map { it.padEnd(k, fill) }

// 9ms
    fun divideString(s: String, k: Int, fill: Char) =
        Array(1 + (s.length - 1) / k) {  s.drop(it * k).take(k).padEnd(k, fill) }

// 8ms
    fun divideString(s: String, k: Int, fill: Char) =
        s.padEnd((1 + (s.length - 1) / k) * k, fill).chunked(k)

// 1ms
    fun divideString(s: String, k: Int, fill: Char): Array<String> {
        val s = s.toCharArray()
        return Array(1 + (s.size - 1) / k) { i ->
            val sz = min(s.size - i * k, k)
            if (sz == k) String(s, i * k, k)
            else {
                val tmp = CharArray(k) { fill };
                System.arraycopy(s, i * k, tmp, 0, sz)
                String(tmp)
            }
        }
    }

// 0ms
    pub fn divide_string(s: String, k: i32, fill: char) -> Vec<String> {
        let mut s = s.chars().collect::<Vec<_>>(); let k = k as usize;
        s.resize((1 + (s.len() - 1) / k) * k, fill);
        s.chunks(k).map(|c| c.iter().collect()).collect()
    }

// 0ms
    vector<string> divideString(string s, int k, char fill) {
        vector<string> r(1 + (size(s) - 1) / k, string(k, fill));
        for (int i = 0; i < size(s); ++i) r[i / k][i % k] = s[i];
        return r;
    }