Adds 'advanced' algorithm solution to challenge (#1218)

* Adds 'advanced' algorithm solution to challenge

closes #1217

* Adds side-effect safe local var to basic and adv

* Adds real side-effect free local var - basic & adv

* Adds ```javascript declaration & update "run code"

Algorithm-Pairwise.md

@@ -44,16 +44,18 @@ It is easy to confuse indices as being numbers, but since you will be interactin
 function pairwise(arr, arg) {
  // Set sum of indices to zero
  var sum = 0;
+ // make a local copy of the arguments object so we don't modify it directly
+ var pairArr = arr.slice();
  // looping from first element
- for(i = 0; i < arr.length; i++) {
+ for(i = 0; i < pairArr.length; i++) {
    //Looping from second element by setting first element  constant
-   for(j = i + 1; j < arr.length; j++) {
+   for(j = i + 1; j < pairArr.length; j++) {
      // Check whether the sum is equal to arg
-     if(arr[i] + arr[j] == arg) {
+     if(pairArr[i] + pairArr[j] == arg) {
        //Add the indices
        sum += i + j;
        //Set the indices to NaN so that they can't be used in next iteration
-       arr[i] = arr[j] = NaN;
+       pairArr[i] = pairArr[j] = NaN;
      }
    }
  }
@@ -148,11 +150,52 @@ pairwise([1,4,2,3,0,5], 7);
 - [JS Array Prototype IndexOf](JS-Array-Prototype-IndexOf)
 - [JS Array Prototype Push](JS-Array-Prototype-Push)
 
+## :rotating_light: Advanced Code Solution:
+
+````javascript
+function pairwise(arr, arg) {
+  // search array for elements that when paired, equal the second argument, then sum their indices
+  // make a local copy of the arguments object so we don't modify it directly
+  var pairArr = arr.slice();
+  return pairArr.reduce( function (a,b,index){ // use native reduce to collect running total of summed indices
+      var search = arg - b; // get difference of current item so we know what value will sum to arg
+
+      // check if search value in rest of the array, but also make sure it doesn't match current search index
+      if ( pairArr.indexOf(search) != -1 && pairArr.indexOf(search) != index ){ 
+         var total = index + pairArr.indexOf(search);  // if found, add to the runnning total
+         pairArr.splice(index,1,NaN); // remove current index from the array
+         pairArr.splice(pairArr.indexOf(search),1,NaN); // remove the other matched element from the array
+         return a + total; //return the running total back to reduce for next item
+      }
+      return a; // simply return previous total if no operations needed
+  },0);
+
+}
+
+// test here
+pairwise([1,4,2,3,0,5], 7);
+````
+
+:rocket: [Run Code](https://repl.it/CLpC/2)
+
+### Code Explanation:
+See comments in code
+This code takes advantage of the fact that the native Array.prototype.indexOf() method will return the lowest index of the value it finds, a requirement of the challenge. Given that you start with the first item in the array (automatically the lowest of it's value), you're guaranteed to always find the lowest pairs, before removing them from the search space.
+
+#### Relevant Links
+- [JS Array Prototype Reduce](JS-Array-Prototype-Reduce)
+- [JS Array Prototype IndexOf](JS-Array-Prototype-IndexOf)
+- [JS Array Prototype Splice](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/splice)
+- [Global NaN property](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/NaN)
+- [JS Reduce Made Easy](JS-Reduce-Made-Easy)
+
+:rocket: [Run Code](https://repl.it/CLpC/1)
+
 ### :trophy: Credits:
 
 If you found this page useful, you may say thanks to the contributors by copying and pasting the following line in the main chat:
 
-**`Thanks @Rafase282 @coded9 @SaintPeter  for your help with Algorithm: Pairwise`**
+**`Thanks @Rafase282 @coded9 @SaintPeter @geligelu for your help with Algorithm: Pairwise`**
 
 ## :clipboard: NOTES FOR CONTRIBUTIONS: