function setCharAt(str, index, chr) {
    if (index > str.length - 1) return str;
    return str.slice(0, index) + chr + str.slice(index + 1);
}

function cavityMap(grid) {
    const len = grid.length;
    for (let x = 1; x < len - 1; x++) {
        for (let y = 1; y < len - 1; y++) {
            let curr = grid[x].charAt(y);
            let compare = [];
            compare.push(grid[x - 1].charAt(y));
            compare.push(grid[x + 1].charAt(y));
            compare.push(grid[x].charAt(y-1));
            compare.push(grid[x].charAt(y + 1));
            if (compare.every(v => v !== "X" && v < curr)) {
                grid[x] = setCharAt(grid[x], y, "X");
            }
        }
    }
    return grid;
}