Feb 1 updated solution

Author: aliasvishnu

41-FirstMissingPositive.cpp

@@ -1,18 +1,43 @@
 class Solution {
 public:
+    // O(n)
     int firstMissingPositive(vector<int>& nums) {
+        nums.push_back(0);
         int len = nums.size();
         if(len == 0) return 1;
 
-        sort(nums.begin(), nums.end());
+        // sort(nums.begin(), nums.end());
+        int i = 0;
+        while(i < len){
+            if( (nums[i] < 0 || nums[i] >= len) || (nums[i] == nums[nums[i]]) ){
+                i++;
+            } else {
+                swap(nums[i], nums[nums[i]]);
+            }
+        }
 
         int answer = 0;
-        for(int i = 0; i < len; i++){
-            if(nums[i] < 0 || nums[i] == answer) continue;
-            if(nums[i] == answer+1) answer = nums[i];
-            else return answer+1;
+        for( i = 1; i < len; i++){
+            if(i != nums[i]) return i;
         }
 
-        return (nums[len-1] < 0)? 1: nums[len-1]+1;
+        return len;
     }
+
+    // O(n log n) solution
+    // int firstMissingPositive(vector<int>& nums) {
+    //     int len = nums.size();
+    //     if(len == 0) return 1;
+    //
+    //     sort(nums.begin(), nums.end());
+    //
+    //     int answer = 0;
+    //     for(int i = 0; i < len; i++){
+    //         if(nums[i] < 0 || nums[i] == answer) continue;
+    //         if(nums[i] == answer+1) answer = nums[i];
+    //         else return answer+1;
+    //     }
+    //
+    //     return (nums[len-1] < 0)? 1: nums[len-1]+1;
+    // }
 };