Repositions index traversal of l

Seems like a good use case for `defer`... giving the index traversal more visibility by putting it next to the `while` loop. Reads a bit more like for loops now, where exit condition and traversal procedure can be understood in 1 line.

Author: kelvinlauKL

3Sum and 4Sum/3Sum.playground/Contents.swift

@@ -32,11 +32,11 @@ func threeSum<T: BidirectionalCollection>(_ collection: T, target: T.Element) ->
   var ret: [[T.Element]] = []
   var l = sorted.startIndex
 
-  while l < sorted.endIndex {
+  while l < sorted.endIndex { defer { sorted.formUniqueIndex(after: &l) }
     var m = sorted.index(after: l)
     var r = sorted.index(before: sorted.endIndex)
 
-    while m < r && r < sorted.endIndex {
+    while m < r && r < sorted.endIndex { 
       let sum = sorted[l] + sorted[m] + sorted[r]
       switch target {
       case sum:
@@ -50,7 +50,6 @@ func threeSum<T: BidirectionalCollection>(_ collection: T, target: T.Element) ->
       default: fatalError("Swift isn't smart enough to detect that this switch statement is exhausive")
       }
     }
-    sorted.formUniqueIndex(after: &l)
   }
 
   return ret