***First Missing Positive (Leetcode 41) (DCP 4)

Problem Link: https://leetcode.com/problems/first-missing-positive/

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        
        n=len(nums)
        
        # Check for 1
        for i in nums:
            if(i==1):
                break
        else:
            return 1
        
        # Change the 0s, -ves and >n's to 1
        # as we have already processed 1 above
        for i in range(n):
            
            if(nums[i]==0 or nums[i]<0 or nums[i]>n):
                nums[i]=1
        
        for i in range(n):
            idx=abs(nums[i])-1
            nums[idx]=-1*abs(nums[idx])
        
        for i in range(n):
            if(nums[i]>0):
                return i+1
        else:
            return n+1