Leetcode 130. Surrounded Regions

class Solution {
public:
    void solve(vector<vector<char>>& board) {
         if(board.size() == 0) {
            return;
        }
        int rows = board.size();
        int columns = board[0].size();
        flipBorder(board, 'O', '1', rows, columns);
        for(int i = 1; i < rows - 1; i++) {
            for(int j = 1; j < columns - 1; j++) {
                if(board[i][j] == 'O') {
                    capture(board, i, j, rows, columns, 'O', 'X');
                }
            }
        }
        flipBorder(board, '1', 'O', rows, columns);

    }
    
private:
    void flipBorder(vector<vector<char>>& board, char src, char target, int& rows, int& columns) {
        for(int i = 0; i < rows; i++) {
            if(board[i][0] == src) {
                capture(board, i, 0, rows, columns, src, target);
            }
            if(board[i][columns - 1] == src) {
                capture(board, i, columns - 1, rows, columns, src, target);
            }
        }
        for(int i = 0; i < columns; i++) {
            if(board[0][i] == src) {
                capture(board, 0, i, rows, columns, src, target);
            }
            if(board[rows - 1][i] == src) {
                capture(board, rows - 1, i, rows, columns, src, target);
            }
        }
    }
    
    void capture(vector<vector<char>>& board, int i, int j, int& rows, int& columns, char src, char target) {
        if(i < 0 || i == rows || j < 0 || j == columns || board[i][j] != src) {
            return;
        }
        board[i][j] = target;
        capture(board, i + 1, j, rows, columns, src, target);
        capture(board, i - 1, j, rows, columns, src, target);
        capture(board, i, j + 1, rows, columns, src, target);
        capture(board, i, j - 1, rows, columns, src, target);
    }
};