Add the solution for finding 'First Missing Positive' number

Author: rayworks

src/main/java/org/sean/array/MissingPositiveFinder.java

@@ -0,0 +1,90 @@
+package org.sean.array;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/***
+ * 41. First Missing Positive
+ * https://leetcode.com/problems/first-missing-positive/
+ */
+public class MissingPositiveFinder {
+    public int firstMissingPositive(int[] nums) { // O(N) time | O(1) space
+        if (nums == null || nums.length == 0) return 1;
+
+        boolean found = false;
+        for (int e : nums) {
+            if (e == 1) {
+                found = true;
+                break;
+            }
+        }
+        if (!found) {
+            return 1;
+        }
+
+        // 1. separate the negatives and zeroes
+        int i = 0, j = nums.length - 1;
+        while (i < j) {
+            while (i < nums.length && nums[i] > 0) i++;
+            while (j >= 0 && nums[j] <= 0) j--;
+
+            if (i >= nums.length || j < 0 || i >= j) break;
+
+            // swap it
+            swap(nums, i, j);
+
+            i++;
+        }
+
+        int lastPos = -1;
+        for (int n = 0; n < nums.length; n++) {
+            if (nums[n] <= 0) {
+                lastPos = n - 1;
+                break;
+            }
+        }
+        if (lastPos == -1) lastPos = nums.length - 1;
+
+        // 2. mark all the present orders
+        for (int k = 0; k <= lastPos; k++) {
+            int index = Math.abs(nums[k]);
+            if (index > 0 && index <= nums.length && nums[index - 1] > 0) { // nums[k] > 0
+                nums[index - 1] = -nums[index - 1];
+            }
+        }
+        for (int m = 0; m <= lastPos; m++) {
+            if (nums[m] > 0) {
+                return m + 1;
+            }
+        }
+
+        // all located : return size + 1 [lastPos + 1] + 1
+        return lastPos + 2;
+    }
+
+    private void swap(int[] nums, int i, int j) {
+        int t = nums[i];
+        nums[i] = nums[j];
+        nums[j] = t;
+    }
+
+    public int firstMissingPositive0(int[] nums) { // O(N) time | O(1) space
+        if (nums == null || nums.length == 0) return 1;
+
+        Set<Integer> set = new HashSet<>();
+
+        for (int i : nums) {
+            if (i > 0) {
+                set.add(i);
+            }
+        }
+
+        for (int k = 1; k <= nums.length + 1; k++) {
+            if (!set.contains(k)) {
+                return k;
+            }
+        }
+
+        return 1;
+    }
+}

src/test/java/org/sean/array/MissingPositiveFinderTest.java

@@ -0,0 +1,24 @@
+package org.sean.array;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class MissingPositiveFinderTest {
+
+    private MissingPositiveFinder finder;
+
+    @Before
+    public void setUp() throws Exception {
+        finder = new MissingPositiveFinder();
+    }
+
+    @Test
+    public void firstMissingPositive() {
+        assertEquals(finder.firstMissingPositive(new int[] {3, 4, -1, 1}), 2);
+        assertEquals(finder.firstMissingPositive(new int[] {1, 2, 0}), 3);
+        assertEquals(finder.firstMissingPositive(new int[] {1, 1}), 2);
+        assertEquals(finder.firstMissingPositive(new int[] {7,8,9,11,12}), 1);
+    }
+}