JS Bin// source https://jsbin.com/woguran

jsbin.woguran.js

// Reverse inplace the two char arrays splitten by space. Examples:
// perfect makes practice -> practice makes perfect
// a b c -> c b a

function reverse(arr) {
  reverseArray(arr, 0, arr.length-1);
}

// reverse array inplace
function reverseArray(arr, start, end) {
  var i = start;
  var j = end;
  var firstSpace;
  var secondSpace;
  
  while (i < j) {
    //console.log(i + ' ' + j + '   [' + arr + ']');
    
    // swap
    swap(arr, i, j);
    
    if (arr[i] === ' ') {
      // reverse first word
      reverseWord(arr, 0, i-1);
      firstSpace = i;
    }

    
    if (arr[j] === ' ') {
      // reverse second word
      reverseWord(arr, j+1, end);
      secondSpace = j;
    } 
    
    i++;
    j--;
  }
  
  // reverse the middle word
  reverseWord(arr, firstSpace, secondSpace);
}

function reverseWord(arr, start, end) {
  var i = start;
  var j = end;
  while (i < j) {
    // swap
    swap(arr, i, j);
    i++;
    j--;
  }
}

function swap(arr, i, j) {
  var tmp = arr[j];
  arr[j] = arr[i];
  arr[i] = tmp;
}

var inputTest = ['a', ' ', 'b', ' ', 'c'];
reverse(inputTest);
console.log(inputTest);
 
var input1 = 'perfect makes practice'.split('');
reverse(input1);
console.log(input1.join(''));

var input2 = 'cat giraffe dog'.split('');
reverse(input2);
console.log(input2.join(''));