Create First_Missing_Positive.java

Author: 17tanya

Arrays/First_Missing_Positive.java

@@ -0,0 +1,51 @@
+//Leetcode 41. First Missing Positive
+//Question - https://leetcode.com/problems/first-missing-positive/
+
+/*Given an unsorted integer array, find the smallest missing positive integer.
+
+Example 1:
+
+Input: [1,2,0]
+Output: 3
+
+Example 2:
+
+Input: [3,4,-1,1]
+Output: 2
+
+Example 3:
+
+Input: [7,8,9,11,12]
+Output: 1
+
+Note:
+
+Your algorithm should run in O(n) time and uses constant extra space.
+*/
+
+class Solution {
+    public int firstMissingPositive(int[] nums) {
+        if(nums==null || nums.length==0) return 1;
+        
+        boolean oneFound = false;
+        for(int i=0;i<nums.length;i++){
+            if(nums[i]==1) oneFound = true;
+            else if(nums[i]<=0) nums[i] = Integer.MAX_VALUE;
+        }
+        
+        if(!oneFound) return 1;
+        
+        for(int i=0 ; i<nums.length ; i++){
+            int targetIndex = Math.abs(nums[i])-1;
+             if(targetIndex>=0 && targetIndex<nums.length){
+                 nums[targetIndex] = Math.abs(nums[targetIndex])*-1;
+             }
+        }
+        
+        for(int i=0;i<nums.length;i++){
+            if(nums[i]>0) return (i+1);
+        }
+        return nums.length+1;
+       
+    }
+}