Reverse a string O(N) and O(N/2)

reverse.js

// O(N)
function(str) {
  if (str.length < 2) return str;
  return str.split('').reverse().join('');
}

// O(N/2)
function(str) {
  var buffer = new Buffer(str);
  for (var i=0;i< buffer.length/2; i++){
    var swap = buffer[i];
    buffer[i] = buffer[buffer.length - 1 - i];
    buffer[buffer.length - 1 - i] = swap;
  }
  return buffer.toString();
}