Reorder_Array_1.js

//Given an input array and another array that describes a new index for each element, 
// mutate the input array so that each element ends up in their new index.
// Discuss the runtime of the algorithm and how you can be sure there won't be any infinite loops
// Input: [a,b,c,d,e,f,g] & [3,6,2,4,0,1,5]
// Output: [e,f,c,a,d,g,b]
// Question to ask: Input validation? Length same? indices array has to contains only numbers from 0 -> length - 1.
function reorderArr(arr, indices){
  //Check if arrays are same length
  if (arr.length !== indices.length || invalidOrder(indices, arr.length - 1)) return;
  
  for (var i = 0; i < arr.length; i++){
    while (indices[i] !== i){
      var newPos = indices[i];
      var savedPos = indices[newPos], savedEl = arr[newPos];
      
      //swap
      indices[newPos] = newPos;
      arr[newPos] = arr[i];
      
      arr[i] = savedEl;
      indices[i] = savedPos;
    }
  }
  
  return arr;
}

function invalidOrder(orders, maxNumber){
  for (var i = 0; i < orders.length; i++){
    var order = orders[i];
    
    if (isNaN(order) || order < 0 || order > maxNumber){
      return false;
    }
  }
  
  return true;
}

Reorder_Array_2.js

//Given an input array and another array that describes a new index for each element, 
// mutate the input array so that each element ends up in their new index.
// Discuss the runtime of the algorithm and how you can be sure there won't be any infinite loops
// Input arr = [8, 6, 7, 5, 3, 0, 9];
// indices = [3, 6, 2, 4, 0, 1, 5];
// Output [5, 9, 7, 3, 8, 6, 0];

function reorderArr2(arr, indices){
  //Check if arrays are same length
  if (arr.length !== indices.length || invalidOrder(indices, arr.length - 1)) return;
  
  for (var i = 0; i < arr.length; i++){
    var element = arr[i];
    var currPos = i;
    
    while (indices[currPos] !== i){
      var nextPos = indices[currPos];
      
      arr[currPos] = arr[nextPos];
      indices[currPos] = currPos;
      
      currPos = nextPos;
    }
    
    arr[currPos] = element;
  }
  
  return arr;
}