add a solution for 1539

Author: fishercoder1534

Diff for: src/main/java/com/fishercoder/solutions/_1539.java

@@ -64,4 +64,36 @@ public int findKthPositive(int[] arr, int k) {
             return result;
         }
     }
+
+    public static class Solution3 {
+        /**
+         * Use binary search:
+         * use an array without missing integers to illustrate:
+         * 1, 2, 3, 4, 5
+         * 2, 3, 4, 7, 11
+         * at index = 2, number of missing positive numbers: arr[index] - index - 1
+         * <p>
+         * Space: O(1)
+         * Time: O(logn)
+         * Credit: https://leetcode.com/problems/kth-missing-positive-number/editorial/
+         */
+        public int findKthPositive(int[] arr, int k) {
+            int left = 0;
+            int right = arr.length - 1;
+            while (left <= right) {
+                int mid = left + (right - left) / 2;
+                if (arr[mid] - mid - 1 < k) {
+                    left = mid + 1;
+                } else {
+                    right = mid - 1;
+                }
+            }
+            //when it exits the above while loop, left = right + 1;
+            //the k-th missing number should be between arr[right] and arr[left]
+            //the number of integers missing before arr[right] is arr[right] - right - 1;
+            //so the number to return is:
+            //arr[right] + k - (arr[right] - right - 1) = k + right + 1 = k + left;
+            return left + k;
+        }
+    }
 }

Diff for: src/test/java/com/fishercoder/_1539Test.java

@@ -1,20 +1,22 @@
 package com.fishercoder;
 
 import com.fishercoder.solutions._1539;
-import org.junit.BeforeClass;
-import org.junit.Test;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
-import static junit.framework.TestCase.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 
 public class _1539Test {
     private static _1539.Solution1 solution1;
     private static _1539.Solution2 solution2;
+    private static _1539.Solution3 solution3;
     private static int[] arr;
 
-    @BeforeClass
-    public static void setup() {
+    @BeforeEach
+    public void setup() {
         solution1 = new _1539.Solution1();
         solution2 = new _1539.Solution2();
+        solution3 = new _1539.Solution3();
     }
 
     @Test
@@ -41,4 +43,16 @@ public void test4() {
         assertEquals(6, solution2.findKthPositive(arr, 2));
     }
 
+    @Test
+    public void test5() {
+        arr = new int[]{2, 3, 4, 7, 11};
+        assertEquals(9, solution3.findKthPositive(arr, 5));
+    }
+
+    @Test
+    public void test6() {
+        arr = new int[]{1, 2, 3, 4};
+        assertEquals(6, solution3.findKthPositive(arr, 2));
+    }
+
 }