Add solution for First Missing Positive in Hashing

21- Hashing/FirstMissingPositive.cpp

@@ -0,0 +1,80 @@
+/*
+
+First Missing Positive (link - https://leetcode.com/problems/first-missing-positive/description/)
+
+Description
+
+Given an unsorted integer array nums, return the smallest missing positive integer.
+
+You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.
+
+
+
+Example 1:
+
+Input: nums = [1,2,0]
+Output: 3
+Explanation: The numbers in the range [1,2] are all in the array.
+
+Example 2:
+
+Input: nums = [3,4,-1,1]
+Output: 2
+Explanation: 1 is in the array but 2 is missing.
+
+Example 3:
+
+Input: nums = [7,8,9,11,12]
+Output: 1
+Explanation: The smallest positive integer 1 is missing.
+
+Constraints:
+
+1 <= nums.length <= 105
+-231 <= nums[i] <= 231 - 1
+
+
+
+*/
+
+
+#include<bits/stdc++.h>
+using namespace std;
+
+// Function to find first positive integer missing from the given list
+int firstMissingPositive(vector<int>& nums) {
+    int n = nums.size();
+    for(int i=0;i<n;i++) {
+        // If number does not point to its location, then swap it with its proper location
+        // Given that it is within the constraints of the length of list and destination does not
+        // contain proper integer as well
+        if(i+1 != nums[i]) {
+            if(nums[i] <= n and nums[i] >= 1 && nums[i] != nums[nums[i]-1]) swap(nums[i], nums[nums[i]-1]),i--;
+        }
+    }
+
+    // Check which location is not containing its respective integer
+    for(int i=0;i<n;i++) {
+        if(i+1 != nums[i]) return i+1;
+    }
+
+    // Return length of list + 1 in case the list contains every positive integer till then
+    return n+1;
+}
+
+// Driver function
+int main() {
+    int n;
+    cout<<"Enter size of list: ";
+    cin>>n;
+    cout<<"Enter Numbers:\n";
+    vector<int> nums;
+    for(int i=0;i<n;i++) {
+        int t;
+        cin>>t;
+        nums.push_back(t);
+    }
+
+    cout<<"First Missing Positive: "<<firstMissingPositive(nums)<<"\n";
+    return 0;
+}