LeetCode Entry

3643. Flip Square Submatrix Vertically

21.03.2026 medium 2026 kotlin rust

Reverse submatrix k by k

3643. Flip Square Submatrix Vertically medium blog post substack youtube

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Problem TLDR

Reverse submatrix k by k #easy

Intuition

Carefully do this in-place.

Approach

• name variables properly
• Rust allows to swap entire slices

Complexity

• Time complexity: \(O(k*k)\)

• Space complexity: \(O(1)\)

Code

// 1ms
    fun reverseSubmatrix(g: Array<IntArray>, r: Int, c: Int, k: Int)=g.also{
        for (y in 0..<k/2) for (x in c..<c+k)
            g[y+r][x] = g[r+k-1-y][x].also { g[r+k-1-y][x] = g[y+r][x] }
    }

// 0ms
    pub fn reverse_submatrix(mut g: Vec<Vec<i32>>, r: i32, c: i32, k: i32) -> Vec<Vec<i32>> {
        let (r, c, k) = (r as usize, c as usize, k as usize);
        let (top, btm) = g[r..r+k].split_at_mut(k/2);
        for (t,b) in top.iter_mut().zip(btm.iter_mut().rev()) {
            t[c..c+k].swap_with_slice(&mut b[c..c+k])
        } g
    }