Solution as on 22-03-2022 08:45 pm

Author: knockcat

0041. First Missing Positive.cpp

@@ -0,0 +1,72 @@
+// 41.✅ First Missing Positive
+
+class Solution
+{
+public:
+    int firstMissingPositive(vector<int> &nums)
+    {
+        set<int> s;
+        int ans = 1;
+        for (int i = 0; i < nums.size(); ++i)
+            s.insert(nums[i]);
+
+        for (int i = 1; i <= nums.size() + 1; ++i)
+        {
+            auto it = s.find(i);
+            if (it == s.end())
+            {
+                ans = i;
+                break;
+            }
+        }
+
+        return ans;
+    }
+};
+
+// Another Approach
+
+class Solution
+{
+public:
+    int firstMissingPositive(vector<int> &nums)
+    {
+        bool contains_one = false;
+        for (int x : nums)
+        {
+            if (x == 1)
+            {
+                contains_one = true;
+                break;
+            }
+        }
+
+        if (!contains_one)
+            return 1;
+
+        int n = nums.size();
+        if (n == 1)
+            return 2;
+
+        for (int i = 0; i < n; ++i)
+        {
+            if (nums[i] <= 0 || nums[i] > n)
+                nums[i] = 1;
+        }
+
+        for (int i = 0; i < n; ++i)
+        {
+            int x = abs(nums[i]);
+            if (nums[x - 1] > 0)
+                nums[x - 1] *= -1;
+        }
+
+        for (int i = 0; i < n; ++i)
+        {
+            if (nums[i] > 0)
+                return i + 1;
+        }
+
+        return n + 1;
+    }
+};