I was solving a question on leetcode. “41. First Missing Positive”

For swapping, when I used nums[i],nums[nums[i]-1] = nums[nums[i]-1],nums[i] , I got TLE.

But nums[nums[i]-1],nums[i] = nums[i],nums[nums[i]-1] is working perfectly.

This is not just the case with leetcode.

I tried to understand the difference, spent 1 hour on the problem.
And then reached out to Google bard and Chatgpt. Both struggled.

After spending lots of time, I got an answer “Given that both codes share the same logic and conditional statements, it remains puzzling why Code 1 is exhibiting an issue, especially in the scenario you described with the infinite loop.”

Here is my code:

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        i,n = 0,len(nums)

        while i<n:
            if nums[i]>0 and nums[i]<=n and nums[nums[i]-1]!=nums[i]:
                #nums[i],nums[nums[i]-1] = nums[nums[i]-1],nums[i]
                nums[nums[i]-1],nums[i] = nums[i],nums[nums[i]-1]

            else:
                i+=1
        for i in range(n):
            if nums[i]!=i+1:
                return i+1
        return n+1
        

The commented out statement is giving TLE , but second one is working fine. Can someone please explain, what is causing this? Is a,b=b,a and b,a=a,b different?

I also defined the function in Python IDLE, added a print statement below the swapping statement, and it got stuck in infinite loop again.

Defined function in Python IDLE

Output (infinite loop)

TLE with first statement

Accepted with second statement