impl Solution {
    pub fn spiral_order(matrix: Vec<Vec<i32>>) -> Vec<i32> {
        // Initialize the result vector to store the spiral order.
        let mut res = vec![];
        
        // Initialize pointers for the current boundaries of the matrix slice we are working on.
        // `l` and `r` for left and right columns, `t` and `b` for top and bottom rows.
        let mut l = 0; // Left boundary
        let mut r = matrix[0].len(); // Right boundary (exclusive)
        let mut t = 0; // Top boundary
        let mut b = matrix.len(); // Bottom boundary (exclusive)

        // Continue looping until the left boundary is less than the right, and the top boundary is less than the bottom.
        // This condition ensures that we are always working within the valid range of the matrix.
        while l < r && t < b {
            // Traverse from left to right on the top row.
            for i in l..r {
                res.push(matrix[t][i]);
            }
            t += 1; // Move the top boundary down as this row is now done.

            // Traverse from top to bottom on the rightmost column.
            for i in t..b {
                res.push(matrix[i][r - 1]);
            }
            r -= 1; // Move the right boundary left as this column is now done.

            // Check if we have covered all rows and columns to prevent double counting the elements in the final row or column.
            if !(l < r && t < b) {
                break;
            }

            // Traverse from right to left on the bottom row.
            for i in (l..r).rev() {
                res.push(matrix[b - 1][i]);
            }
            b -= 1; // Move the bottom boundary up as this row is now done.

            // Traverse from bottom to top on the leftmost column.
            for i in (t..b).rev() {
                res.push(matrix[i][l]);
            }
            l += 1; // Move the left boundary right as this column is now done.
        }

        // Return the result vector containing the elements of the matrix in spiral order.
        res
    }
}