# First Missing Positive – Leetcode Python

LeetCode has a Hard coding Problem in Its’ Algorithm Section “First Missing Positive”. Today We are going to solve this problem. LeetCode Link of the Problem is HERE

#### Question

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 constant extra space.

Examples

```Input: nums = [1,2,0]
Output: 3
```
```Input: nums = [3,4,-1,1]
Output: 2
```
```Input: nums = [7,8,9,11,12]
Output: 1
```

Constraints:

• `1 <= nums.length <= 5 * 105`
• `-231 <= nums[i] <= 231 - 1`

#### Solution to First Missing Positive

The Skeleton Code given by LeetCode is

``````class Solution(object):
def firstMissingPositive(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
``````

Complete Solution:

``````class Solution(object):
def firstMissingPositive(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
n = len(nums)
for i in range(n):
# To determine whether this number can be exchanged
while nums[i]>0 and nums[i]<=n and nums[i]!=nums[nums[i]-1]:
self.swap(nums,i,nums[i]-1)
for i in range(n):
if nums[i]!=i+1:
return i+1
return n+1
def swap(self,nums,i,j):
nums[i],nums[j] = nums[j],nums[i]``````

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