add problem 41 and test function

Author: supercoderhawk

C/1-50/41-First-Missing-Positive.c

@@ -0,0 +1,32 @@
+/*
+ * Given an unsorted integer array, find the first missing positive integer.
+
+ * For example,
+ * Given [1,2,0] return 3,
+ * and [3,4,-1,1] return 2.
+
+ * Your algorithm should run in O(n) time and uses constant space.
+ * Created by supercoderx on 2017/8/13.
+ */
+#include 
+
+int firstMissingPositive(int *nums, int numsSize) {
+    int tmp;
+    for (int i = 0; i < numsSize; ++i) {
+        while (nums[i] > 0 && nums[i] <= numsSize && nums[nums[i] - 1] != nums[i]) {
+            tmp = nums[i];
+            nums[i] = nums[tmp - 1];
+            nums[tmp - 1] = tmp;
+        }
+    }
+    for (int j = 0; j < numsSize; ++j) {
+        if (nums[j] != j + 1)
+            return j + 1;
+    }
+    return numsSize + 1;
+}
+
+void testFirstMissingPositive() {
+    int a[] = {2, 1};
+    printf("%d", firstMissingPositive(a, 2));
+}