Add MissingNumber task solution

Author: andrei-punko

src/main/java/by/andd3dfx/numeric/MissingNumber.java

@@ -0,0 +1,43 @@
+package by.andd3dfx.numeric;
+
+/**
+ * <pre>
+ * <a href="https://leetcode.com/problems/missing-number/description/">Task description</a>
+ *
+ * Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.
+ *
+ *  Example 1:
+ * Input: nums = [3,0,1]
+ * Output: 2
+ * Explanation:
+ * n = 3 since there are 3 numbers, so all numbers are in the range [0,3]. 2 is the missing number in the range since it does not appear in nums.
+ *
+ * Example 2:
+ * Input: nums = [0,1]
+ * Output: 2
+ * Explanation:
+ * n = 2 since there are 2 numbers, so all numbers are in the range [0,2]. 2 is the missing number in the range since it does not appear in nums.
+ *
+ * Example 3:
+ * Input: nums = [9,6,4,2,3,5,7,0,1]
+ * Output: 8
+ * Explanation:
+ * n = 9 since there are 9 numbers, so all numbers are in the range [0,9]. 8 is the missing number in the range since it does not appear in nums.
+ * </pre>
+ */
+public class MissingNumber {
+
+    public int missingNumber(int[] nums) {
+        var n = nums.length;
+        var flags = new boolean[n + 1];
+        for (var num : nums) {
+            flags[num] = true;
+        }
+        for (var i = 0; i <= n; i++) {
+            if (!flags[i]) {
+                return i;
+            }
+        }
+        return -1;
+    }
+}

src/test/java/by/andd3dfx/numeric/MissingNumberTest.java

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+package by.andd3dfx.numeric;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class MissingNumberTest {
+
+    private final MissingNumber missingNumber = new MissingNumber();
+
+    @Test
+    public void testMissingNumber() {
+        assertThat(missingNumber.missingNumber(new int[]{3, 0, 1})).isEqualTo(2);
+        assertThat(missingNumber.missingNumber(new int[]{0, 1})).isEqualTo(2);
+        assertThat(missingNumber.missingNumber(new int[]{9, 6, 4, 2, 3, 5, 7, 0, 1})).isEqualTo(8);
+    }
+}