3643. Flip Square Submatrix Vertically

You are given an m x n integer matrix grid, and three integers x, y, and k.

The integers x and y represent the row and column indices of the top-left corner of a square submatrix and the integer k represents the size (side length) of the square submatrix.

Your task is to flip the submatrix by reversing the order of its rows vertically.

Return the updated matrix.

class Solution {
public:
    vector<vector<int>> reverseSubmatrix(vector<vector<int>>& grid, int x, int y, int k) {
        for(int i = x, j = x + k - 1; i < j; i++,j--) {
            for(int pos = y; pos < y + k; pos++) {
                swap(grid[i][pos], grid[j][pos]);
            }
        }
        return grid;
    }
};