added first missing positive number progrma

Author: smv1999

Arrays/first_missing_positive.cpp

@@ -0,0 +1,58 @@
+/*
+Given an array of integers, find the first missing positive integer in linear time and constant space. In other words, 
+find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well.
+
+For example, the input [3, 4, -1, 1] should give 2. The input [1, 2, 0] should give 3.
+
+You can modify the input array in-place.
+
+*/
+
+
+#include<iostream>
+#include<cstdlib>
+#include<algorithm>
+#include<conio.h>
+
+using namespace std;
+
+// function prototype
+void printArray(int arr[], int n);
+int firstMissingNumber(int arr[], int n);
+
+
+int firstMissingNumber(int arr[], int n)
+{
+    int index, temp;
+    for(index=0;index<n;index++)
+    {
+        while(arr[index]!=arr[arr[index]-1] && arr[index]>=1 && arr[index]<=n)
+        {
+            temp = arr[arr[index]-1];
+            arr[arr[index]-1] = arr[index];
+            arr[index] = temp;
+
+        }
+    }
+    for(index=0;index<n;index++)
+    {
+        if((index+1)!=arr[index])
+            return index+1;
+    }
+    return n+1;
+}
+int main()
+{
+    int n;
+    cin>>n;
+    int arr[n], index, res;
+    for(index=0;index<n;index++)
+        cin>>arr[index];
+    // finding the first positive missing number
+    printf("\n");
+    res = firstMissingNumber(arr,n);
+    cout<<res;
+    return 0;
+}
+
+