Adjacency matrix to adjacency list

def matrix_to_list(matrix):
    # Row i lists the edges leaving i; reading it left to right keeps them sorted.
    return [[j for j, x in enumerate(row) if x == 1] for row in matrix]

#include <vector>

std::vector<std::vector<int>> solve(const std::vector<std::vector<int>>& matrix) {
    const int n = matrix.size();
    std::vector<std::vector<int>> adj(n);
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < n; ++j)
            if (matrix[i][j] == 1) adj[i].push_back(j);   // row i: edges leaving i
    return adj;
}

class Solution {
    public int[][] matrixToList(int[][] matrix) {
        int n = matrix.length;
        int[][] adj = new int[n][];
        for (int i = 0; i < n; i++) {
            int count = 0;
            for (int x : matrix[i]) if (x == 1) count++;
            adj[i] = new int[count];
            int k = 0;
            for (int j = 0; j < n; j++) if (matrix[i][j] == 1) adj[i][k++] = j;   // row i: edges leaving i
        }
        return adj;
    }
}

function matrixToList(matrix) {
  // Row i lists the edges leaving i; reading it left to right keeps them sorted.
  return matrix.map(row => {
    const out = [];
    for (let j = 0; j < row.length; j++) if (row[j] === 1) out.push(j);
    return out;
  });
}

fn solve<const N: usize>(matrix: &[[i64; N]]) -> Vec<Vec<i64>> {
    // Row i lists the edges leaving i; reading it left to right keeps them sorted.
    matrix
        .iter()
        .map(|row| (0..N).filter(|&j| row[j] == 1).map(|j| j as i64).collect())
        .collect()
}

solve :: [[Int]] -> [[Int]]
-- Row i lists the edges leaving i; reading it left to right keeps them sorted.
solve matrix = [[j | (j, x) <- zip [0 ..] row, x == 1] | row <- matrix]