Added task 23.

Author: javadev

src/main/java/s0023.merge.k.sorted.lists/ListNode.java

@@ -0,0 +1,34 @@
+package s0023.merge.k.sorted.lists;
+
+public class ListNode {
+    int val;
+    ListNode next;
+
+    ListNode() {}
+
+    ListNode(int val) {
+        this.val = val;
+    }
+
+    ListNode(int val, ListNode next) {
+        this.val = val;
+        this.next = next;
+    }
+
+    public String toString() {
+        String result = "" + val;
+        if (next.next != null) {
+            result += ", ";
+        }
+        ListNode current = next;
+        while (current.next != null) {
+            result += current.val;
+            if (current.next != null) {
+                result += ", ";
+            }
+            current = current.next;
+        }
+        result += current.val;
+        return result;
+    }
+}

src/main/java/s0023.merge.k.sorted.lists/Solution.java

@@ -0,0 +1,52 @@
+package s0023.merge.k.sorted.lists;
+
+public class Solution {
+    public ListNode mergeKLists(ListNode[] lists) {
+        if (lists.length == 0) return null;
+        return mergeKLists(lists, 0, lists.length);
+    }
+
+    private ListNode mergeKLists(ListNode[] lists, int leftIndex, int rightIndex) {
+        if (rightIndex > leftIndex + 1) {
+            int mid = (leftIndex + rightIndex) / 2;
+            ListNode left = mergeKLists(lists, leftIndex, mid);
+            ListNode right = mergeKLists(lists, mid, rightIndex);
+            return mergeTwoLists(left, right);
+        } else {
+            return lists[leftIndex];
+        }
+    }
+
+    private ListNode mergeTwoLists(ListNode left, ListNode right) {
+        if (left == null) return right;
+        if (right == null) return left;
+        ListNode res = null;
+        if (left.val <= right.val) {
+            res = left;
+            left = left.next;
+        } else {
+            res = right;
+            right = right.next;
+        }
+        ListNode node = res;
+        while (left != null || right != null) {
+            if (left == null) {
+                node.next = right;
+                right = right.next;
+            } else if (right == null) {
+                node.next = left;
+                left = left.next;
+            } else {
+                if (left.val <= right.val) {
+                    node.next = left;
+                    left = left.next;
+                } else {
+                    node.next = right;
+                    right = right.next;
+                }
+            }
+            node = node.next;
+        }
+        return res;
+    }
+}

src/test/java/s0023.merge.k.sorted.lists/SolutionTest.java

@@ -0,0 +1,43 @@
+package s0023.merge.k.sorted.lists;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import java.util.Arrays;
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void mergeKLists() {
+        ListNode head = new ListNode();
+        ListNode node1 = new ListNode();
+        node1.val = 1;
+        head.next = node1;
+        ListNode node2 = new ListNode();
+        node2.val = 4;
+        node1.next = node2;
+        ListNode node3 = new ListNode();
+        node3.val = 5;
+        node2.next = node3;
+        ListNode head2 = new ListNode();
+        ListNode node4 = new ListNode();
+        node4.val = 1;
+        head2.next = node4;
+        ListNode node5 = new ListNode();
+        node5.val = 3;
+        node4.next = node5;
+        ListNode node6 = new ListNode();
+        node6.val = 4;
+        node5.next = node6;
+        ListNode head3 = new ListNode();
+        ListNode node7 = new ListNode();
+        node7.val = 2;
+        head3.next = node7;
+        ListNode node8 = new ListNode();
+        node8.val = 6;
+        node7.next = node8;
+        assertThat(
+                new Solution().mergeKLists(new ListNode[] {node1, node4, node7}).toString(),
+                equalTo("1, 1, 2, 3, 4, 4, 5, 6"));
+    }
+}