2.5 - add recursion solution and bonus problem (careercup#30)

* adding an example that is slightly less performant but easier to read.

* adding an even better recursion solution

* creating recursive examples of exercise 2.5 and the added the forward solution

* made some formatting changes for the recursions algorithm

Author: bialesdaniel

chapter02/2.5 - Sum Lists/sumLists - forward.js

@@ -0,0 +1,83 @@
+const LinkedList = require('./../util/LinkedList')
+const printList = require('./../util/printList')
+
+function sumLinkedListsForward(list1, list2) {
+  if (!list1 && !list2) {
+    return null
+  }
+  let length1 = length(list1)
+  let length2 = length(list2)
+
+  if (length1 > length2) {
+    list2 = padList(list2, length1 - length2)
+  } else if (length1 < length2) {
+    list1 = padList(list1, length2 - length1)
+  }
+
+  const { head, nextDigitValue } = carryBase10(sumAndAppendNodes(list1, list2), 0)
+  return nextDigitValue ? appendToStart(head, new LinkedList(nextDigitValue)) : head
+}
+
+function length(node) {
+  let count = 0
+  while (node) {
+    count++
+    node = node.next
+  }
+  return count
+}
+
+function padList(shortList, padding) {
+  while (padding > 0) {
+    shortList = appendToStart(shortList, new LinkedList(0))
+    padding--
+  }
+  return shortList
+}
+
+function appendToStart(head, node) {
+  node.next = head
+  return node
+}
+
+function sumAndAppendNodes(node1, node2) {
+  let value = (node1 ? node1.value : 0) + (node2 ? node2.value : 0)
+  if (!node1.next && !node2.next) {
+    return new LinkedList(value)
+  }
+  const {
+    head,
+    nextDigitValue
+  } = carryBase10(sumAndAppendNodes(node1.next, node2.next), value)
+  return appendToStart(head, new LinkedList(nextDigitValue))
+}
+
+function carryBase10(head, nextDigitValue) {
+  if (head.value >= 10) {
+    head.value = head.value % 10
+    nextDigitValue += 1
+  }
+  return {
+    head,
+    nextDigitValue
+  }
+}
+
+// Input: (6 -> 1 -> 7) + (2 -> 9 -> 5). this case refers to 617 + 295
+// Output: 9 -> 1 -> 2. the answer refers to 912
+
+var a = new LinkedList(6)
+var b = new LinkedList(1)
+var c = new LinkedList(7)
+
+a.next = b
+b.next = c
+
+var d = new LinkedList(2)
+var e = new LinkedList(9)
+var f = new LinkedList(5)
+
+d.next = e
+e.next = f
+
+printList(sumLinkedListsForward(a, d))

chapter02/2.5 - Sum Lists/sumLists - recursion.js

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+const LinkedList = require('./../util/LinkedList')
+const printList = require('./../util/printList')
+
+function sumLinkedLists(node1, node2, carry=0){
+  if(!node1 && !node2 && carry===0){
+    return null
+  }
+  let value = carry
+  value += node1 ? node1.value : 0
+  value += node2 ? node2.value : 0
+  const node  = new LinkedList(value%10)
+  node.next = sumLinkedLists(
+    node1 ? node1.next : null,
+    node2 ? node2.next : null,
+    value > 10 ? 1 : 0)
+  return node
+}
+
+// Input: (7 -> 1 -> 6) + (5 -> 9 -> 2). this case refers to 617 + 295
+// Output: 2 -> 1 -> 9. the answer refers to 912
+
+var a = new LinkedList(7)
+var b = new LinkedList(1)
+var c = new LinkedList(6)
+
+a.next = b
+b.next = c
+
+var d = new LinkedList(5)
+var e = new LinkedList(9)
+var f = new LinkedList(2)
+
+d.next = e
+e.next = f
+
+printList(sumLinkedLists(a,d))