Surrounding regions using flash flood algorithm

Set all of the O to – across the board

For each side find adjacent – and change them to Os using flash flood algorithm

Change all of the remaining – to X

The complexity of the above solution is O(m*n)

/**
 * @param {character[][]} board
 * @return {void} Do not return anything, modify board in-place instead.
 */
var solve = function(board) {   
     for (let i=0; i< board.length; i++) {
        for (let j=0; j< board[i].length; j++){ 
            if (board[i][j] == 'O') {
                board[i][j] = '-';
            }
        }
     }
       
    //left side
    for (let i=0; i< board.length; i++) {
        floodFill(board, i, 0, '-', 'O' );
    }
      //right side
    for (let i=0; i< board.length; i++) {
        floodFill(board, i, board[i].length-1, '-', 'O' );
    }
        //top side
    for (let j=0; j< board[0].length; j++) {
        floodFill(board, 0, j, '-', 'O' );
    }
    //bottom side
    for (let j=0; j< board[0].length; j++) {
        floodFill(board, board.length-1, j, '-', 'O' );
    }
    
     for (let i=0; i< board.length; i++) {
        for (let j=0; j< board[i].length; j++){ 
            if (board[i][j] == '-') {
                board[i][j] = 'X';
            }
        }
     }
    
    return board;
};


var floodFill = function(board, i, j, prevVal, newVal) {
    if (i<0 || j<0 || i>=board.length || j>=board[0].length) {
        return;
    }
    
    if (prevVal != board[i][j]) {
        return;
    }
     
    board[i][j] = newVal;
   
    //check north
     floodFill(board, i-1,j, prevVal, newVal);
    //check east
    floodFill(board, i,j+1, prevVal, newVal);
    //check south 
    floodFill(board, i+1,j, prevVal, newVal);
    //check west
    floodFill(board, i,j-1, prevVal, newVal);
  
}