diff --git a/reductions/src/main/scala/reductions/LineOfSight.scala b/reductions/src/main/scala/reductions/LineOfSight.scala
index fa57939..49cb5dc 100644
--- a/reductions/src/main/scala/reductions/LineOfSight.scala
+++ b/reductions/src/main/scala/reductions/LineOfSight.scala
@@ -41,13 +41,29 @@ case class Leaf(from: Int, until: Int, maxPrevious: Float) extends Tree
 object LineOfSight extends LineOfSightInterface {
 
   def lineOfSight(input: Array[Float], output: Array[Float]): Unit = {
-    ???
+    var i = 1
+    var max: Float = 0
+
+    output(0) = 0
+    while (i < input.length) {
+      max = Math.max(max, input(i) / i)
+      output(i) = max
+      i += 1
+    }
   }
 
   /** Traverses the specified part of the array and returns the maximum angle.
    */
   def upsweepSequential(input: Array[Float], from: Int, until: Int): Float = {
-    ???
+    var i = from
+    var max: Float = 0
+
+    while (i < until) {
+      max = Math.max(max, input(i) / i)
+      i += 1
+    }
+
+    max
   }
 
   /** Traverses the part of the array starting at `from` and until `end`, and
@@ -60,7 +76,17 @@ object LineOfSight extends LineOfSightInterface {
    */
   def upsweep(input: Array[Float], from: Int, end: Int,
     threshold: Int): Tree = {
-    ???
+    if (end - from <= threshold) {
+      Leaf(from, end, upsweepSequential(input, from, end))
+    } else {
+      val mid: Int = (end - from) / 2
+      val (l, r) = parallel(
+        upsweep(input, from, from + mid, threshold),
+        upsweep(input, from + mid, end, threshold),
+      )
+
+      Node(l, r)
+    }
   }
 
   /** Traverses the part of the `input` array starting at `from` and until
@@ -69,7 +95,14 @@ object LineOfSight extends LineOfSightInterface {
    */
   def downsweepSequential(input: Array[Float], output: Array[Float],
     startingAngle: Float, from: Int, until: Int): Unit = {
-    ???
+    var i = from
+    var max = startingAngle
+
+    while (i < until) {
+      max = Math.max(max, input(i) / i)
+      output(i) = max
+      i += i
+    }
   }
 
   /** Pushes the maximum angle in the prefix of the array to each leaf of the
@@ -77,13 +110,19 @@ object LineOfSight extends LineOfSightInterface {
    *  the `output` angles.
    */
   def downsweep(input: Array[Float], output: Array[Float], startingAngle: Float,
-    tree: Tree): Unit = {
-    ???
+    tree: Tree): Unit = tree match {
+    case Leaf(from, until, _) => downsweepSequential(input, output, startingAngle, from, until)
+    case Node(l, r) => parallel(
+      downsweep(input, output, startingAngle, l),
+      downsweep(input, output, Math.max(startingAngle, l.maxPrevious), r),
+    )
   }
 
   /** Compute the line-of-sight in parallel. */
   def parLineOfSight(input: Array[Float], output: Array[Float],
     threshold: Int): Unit = {
-    ???
+    val t = upsweep(input, 1, input.length, threshold)
+    downsweep(input, output, 0, t)
+    output(0) = 0
   }
 }
diff --git a/reductions/src/main/scala/reductions/ParallelCountChange.scala b/reductions/src/main/scala/reductions/ParallelCountChange.scala
index 4c749c7..067e6a8 100644
--- a/reductions/src/main/scala/reductions/ParallelCountChange.scala
+++ b/reductions/src/main/scala/reductions/ParallelCountChange.scala
@@ -50,8 +50,16 @@ object ParallelCountChange extends ParallelCountChangeInterface {
   /** Returns the number of ways change can be made from the specified list of
    *  coins for the specified amount of money.
    */
-  def countChange(money: Int, coins: List[Int]): Int = {
-    ???
+  def countChange(money: Int, coins: List[Int]): Int = money match {
+    case 0 => 1
+    case _ => coins match {
+      case Nil => 0
+      case c::cs =>
+        if (money < c) 0
+        else {
+          countChange(money - c, coins) + countChange(money, cs)
+        }
+    }
   }
 
   type Threshold = (Int, List[Int]) => Boolean
@@ -60,20 +68,29 @@ object ParallelCountChange extends ParallelCountChangeInterface {
    *  specified list of coins for the specified amount of money.
    */
   def parCountChange(money: Int, coins: List[Int], threshold: Threshold): Int = {
-    ???
+    if (threshold(money, coins) || money < 0 || coins == Nil) {
+      countChange(money, coins)
+    } else {
+      val (countLeft, countRight) = parallel(
+        parCountChange(money - coins.head, coins, threshold),
+        parCountChange(money, coins.tail, threshold)
+      )
+
+      countLeft + countRight
+    }
   }
 
   /** Threshold heuristic based on the starting money. */
   def moneyThreshold(startingMoney: Int): Threshold =
-    ???
+    (money, _) => money <= 2 * startingMoney / 3
 
   /** Threshold heuristic based on the total number of initial coins. */
   def totalCoinsThreshold(totalCoins: Int): Threshold =
-    ???
+    (_, coins) => coins.length <= 2 * totalCoins / 3
 
 
   /** Threshold heuristic based on the starting money and the initial list of coins. */
   def combinedThreshold(startingMoney: Int, allCoins: List[Int]): Threshold = {
-    ???
+    (money, coins) => money * coins.length <= startingMoney * allCoins.length / 2
   }
 }
diff --git a/reductions/src/main/scala/reductions/ParallelParenthesesBalancing.scala b/reductions/src/main/scala/reductions/ParallelParenthesesBalancing.scala
index 21c1757..a54851f 100644
--- a/reductions/src/main/scala/reductions/ParallelParenthesesBalancing.scala
+++ b/reductions/src/main/scala/reductions/ParallelParenthesesBalancing.scala
@@ -40,22 +40,45 @@ object ParallelParenthesesBalancing extends ParallelParenthesesBalancingInterfac
   /** Returns `true` iff the parentheses in the input `chars` are balanced.
    */
   def balance(chars: Array[Char]): Boolean = {
-    ???
+    val z: (Int, Boolean) = (0, true)
+    val (s, alwaysAboveZero) = chars.foldLeft(z)({ case ((s, alwaysAboveZero), c) => c match {
+      case '(' => (s + 1, alwaysAboveZero)
+      case ')' => (s - 1, if (s - 1 < 0) false else alwaysAboveZero)
+      case _ => (s, alwaysAboveZero)
+    }})
+
+    alwaysAboveZero && s == 0
   }
 
   /** Returns `true` iff the parentheses in the input `chars` are balanced.
    */
   def parBalance(chars: Array[Char], threshold: Int): Boolean = {
 
-    def traverse(idx: Int, until: Int, arg1: Int, arg2: Int) /*: ???*/ = {
-      ???
+    @tailrec
+    def traverse(idx: Int, until: Int, arg1: Int, arg2: Int): (Int, Int) = {
+      if (idx == until) (arg1, arg2)
+      else chars(idx) match {
+        case '(' => traverse(idx + 1, until, arg1 + 1, arg2)
+        case ')' => traverse(idx + 1, until, arg1 - 1, Math.min(arg1 - 1, arg2))
+        case _ => traverse(idx + 1, until, arg1, arg2)
+      }
     }
 
-    def reduce(from: Int, until: Int) /*: ???*/ = {
-      ???
+    def reduce(from: Int, until: Int): (Int, Int) = {
+      if (until - from <= threshold) {
+        traverse(from, until, 0, 0)
+      } else {
+        val mid: Int = (until - from) / 2
+        val ((c1, min1), (c2, min2)) = parallel(
+          reduce(from, from + mid),
+          reduce(from + mid, until)
+        )
+
+        (c1 + c2, Math.min(min1, c1 + min2))
+      }
     }
 
-    reduce(0, chars.length) == ???
+    reduce(0, chars.length) == (0, 0)
   }
 
   // For those who want more: