41. First Missing Positive

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41. First Missing Positive

Description

Given an unsorted integer array nums. Return the smallest positive integer that is not present in nums.

You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.

Example 1:

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

Example 2:

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

Example 3:

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

Constraints:

  • 1 <= nums.length <= 10^5
  • -2^31 <= nums[i] <= 2^31 - 1

Hints/Notes

Solution

Language: C++

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class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        int n = nums.size();
        for (int i = 0; i < n; i++) {
            while (nums[i] != i + 1) {
                if (nums[i] <= 0 || nums[i] > n || nums[i] == nums[nums[i] - 1]) {
                    break;
                }
                // nums[i] should be put on index nums[i] - 1
                int idx = nums[i] - 1;
                swap(nums[idx], nums[i]);
            }
        }
        for (int i = 0; i < n; i++) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return n + 1;
    }
};