[Feature] Added first missing positive algorithm.

Author: exterkamp

algorithms/math/first_missing_positive.py

@@ -0,0 +1,18 @@
+def first_missing_positive(nums):
+    """
+    :type nums: List[int]
+    :rtype: int
+    """
+    start = 0
+    end = len(nums) - 1
+    while start <= end:
+        i = nums[start] -1
+        if i == start:
+            start += 1
+        elif i < 0 or i > end or nums[start] == nums[i]:
+            nums[start] = nums[end]
+            end -= 1
+        else:
+            nums[start] = nums[i]
+            nums[i] = i + 1
+    return start + 1

tests/test_first_missing_positive.py

@@ -0,0 +1,14 @@
+import unittest
+from algorithms.math.first_missing_positive import first_missing_positive
+
+class TestFirstMissingPositive(unittest.TestCase):
+
+    def test_in_order(self):
+        self.assertEqual(6,first_missing_positive([1,2,3,4,5]))
+        self.assertEqual(6,first_missing_positive([0,1,2,3,4,5]))
+    
+    def test_with_negatives(self):
+        self.assertEqual(7,first_missing_positive([-1,0,-3,6,-5,2,5,3,4,-9,0,1]))
+    
+    def test_filled_with_zeros(self):
+        self.assertEqual(3,first_missing_positive([0,0,0,1,0,0,0,0,0,2,0,0,0,0]))