linked list practice

src/Exercises.java

@@ -12,7 +12,14 @@ public class Exercises {
      * @return sum of the values in the list
      */
     public static int sum(ListNode head) {
-        return -1;
+        int total = 0;
+
+        while(head != null){
+            total += head.data;
+            head = head.next;
+        }
+
+        return total;
     }
 
     /**
@@ -28,7 +35,16 @@ public static int sum(ListNode head) {
      * @return a count of the negative values in the list
      */
     public static int countNegative(ListNode head) {
-        return -1;
+
+        int count = 0;
+
+        while(head != null){
+            if(head.data < 0){
+                count++;
+            }
+            head = head.next;
+        }
+        return count;
     }
 
     /**
@@ -46,6 +62,16 @@ public static int countNegative(ListNode head) {
      * @param toAdd the value to append in a new node
      */
     public static void addToEnd(ListNode head, int toAdd) {
+        if (head == null){
+            return;
+        }
+
+        ListNode current = head;
+        while (current.next != null){
+            current = current.next;
+        }
+
+        current.next = new ListNode(toAdd);
 
     }
 
@@ -63,6 +89,18 @@ public static void addToEnd(ListNode head, int toAdd) {
      */
     public static void makePositive(ListNode head) {
 
+        if (head == null){
+            return;
+        }
+
+        ListNode current = head;
+        while(current != null){
+            if(current.data < 0){
+                current.data = -current.data;
+            }
+            current = current.next;
+        }
+
     }
 
     /**
@@ -82,6 +120,19 @@ public static void makePositive(ListNode head) {
      * @return whether the list is increasing
      */
     public static boolean isIncreasing(ListNode head) {
-        return false;
+
+        if (head == null || head.next == null){
+            return true;
+        }
+
+        ListNode current = head;
+
+        while(current.next != null){
+            if(current.data > current.next.data){
+                return false;
+            }
+            current = current.next;
+        }
+        return true;
     }
 }