LeetCode Entry

1861. Rotating the Box

06.05.2026 medium 2026 kotlin rust

Rotate and simulate gravity in matrix

1861. Rotating the Box medium substack youtube

https://dmitrysamoylenko.com/leetcode/

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Problem TLDR

Rotate and simulate gravity in matrix

Intuition

• first simulate ‘.’ bubble left or ‘#’ bubble right
• then rotate

Approach

• to bubble ‘.’ left: overwrite it with ‘#’ then overwrite empty place with ‘.’
• another solution is to split by ‘*’ chunks and sort them

Complexity

• Time complexity: \(O(n|nlogn)\)

• Space complexity: \(O(n)\)

Code

    fun rotateTheBox(b: Array<CharArray>) = b.map { r ->
        var j = 0
        for (i in r.indices)
            if (r[i] == '.') { r[i] = '#'; r[j++] = '.' }
            else if (r[i] == '*') j = i+1
    }.let { List(b[0].size) { i -> List(b.size) { b[b.size-1-it][i] }} }

    pub fn rotate_the_box(mut b: Vec<Vec<char>>) -> Vec<Vec<char>> {
        for r in &mut b {
            for c in r.split_mut(|&c|c=='*') { c.sort_by(|a,b|b.cmp(a)) }};
        (0..b[0].len()).map(|x|b.iter().rev().map(|r|r[x]).collect()).collect()
    }