Added Java Solution for First_missing_positive

Author: 2000shivam659

LeetCode/First Missing Positive.java

@@ -0,0 +1,58 @@
+// Question : https://leetcode.com/problems/first-missing-positive/
+
+//Example :
+/**
+ * Example 1:
+ * Input: nums = [1,2,0]
+ * Output: 3
+ * 
+ * Example 2:
+ * nput: nums = [3,4,-1,1]
+ * Output: 2
+ */
+
+ // Solution :
+
+import java.*;
+import java.util.Scanner;
+
+class Solution {
+    public static int firstMissingPositive(int[] nums) {
+        int n = nums.length;
+
+        // First of all sort the array using cyclic sort
+        
+        for(int i = 0; i < n; i++)
+            while(nums[i] > 0 && nums[i] <= n && nums[nums[i] - 1] != nums[i]) {
+                int temp1 = nums[i];
+                int temp2 = nums[nums[i] - 1];
+                nums[i] = te;
+                nums[temp1 - 1] = temp1;
+            }
+
+        // Find out first missing positive integer from sorted nums.
+
+        for(int j = 0; j < n; j++)
+            if(nums[j] != j + 1)
+                return j + 1;
+        
+        return n + 1;
+    }
+
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+
+        int n = sc.nextInt();
+        int[] nums = new int[n];
+
+        for(int i = 0; i < n ; i ++)
+            nums[i] = sc.nextInt();
+
+        System.out.println(firstMissingPositive(nums));
+
+        sc.close();
+    }
+}
+
+// Time complexity : O(n)
+// Space complexity : O(1)