reverseString = function(s) {
    let i = 0, j = s.length-1;
    
    while(i <= j) {
        let leftval = s[i], rightval = s[j];
        s[i] = rightval;
        s[j] = leftval;
        
        i++;
        j--;
    }
}
@param {character[]} s
@return {void} Do not return anything, modify s in-place instead.