# [Solved] 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.

## 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.

Example 1:

Input: nums = [1,2,0]
Output: 3
Explanation: The numbers in the range [1,2] are all in the array.

Example 2:

Input: nums = [3,4,-1,1]
Output: 2
Explanation: 1 is in the array but 2 is missing.

Example 3:

Input: nums = [7,8,9,11,12]
Output: 1
Explanation: The smallest positive integer 1 is missing.

Constraints:

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

## Python Solution

class Solution:
def firstMissingPositive(self, nums: List[int]) -> int:
d = {}
l = len(nums)
if nums == []:
return 1
if nums==[1]:
return 2

for i in range(l):
d[nums[i]] = i

for i in range(1,l+1):
if i not in d:
if i==None:
break
else:
return i
return l+1