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41. First Missing Positive

Difficulty Topics

Description

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

Solution

first-missing-positive.py
class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:

        n = len(nums)

        for i in range(n):
            while (nums[i] > 0 and nums[i] <= n) and nums[nums[i]-1] != nums[i]:
                nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]

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

        return n + 1
first-missing-positive.cpp
class Solution{
public:
    int firstMissingPositive(vector<int>& A)
    {
        int n = A.size();
        for(int i = 0; i < n; ++ i)
            while(A[i] > 0 && A[i] <= n && A[A[i] - 1] != A[i])
                swap(A[i], A[A[i] - 1]);

        for(int i = 0; i < n; ++ i)
            if(A[i] != i + 1)
                return i + 1;

        return n + 1;
    }
};