Link to Question

HARD

First Missing Positive

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

Input:

nums = [1,2,0]

Output:

3

Explanation:
The numbers 1 and 2 are present, so the first missing positive is 3.

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Constraints

• 1 ≤ nums.length ≤ 10⁵
• -2³¹ ≤ nums[i] ≤ 2³¹ - 1

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Solution: In-Place Hashing

• Time Complexity: O(n)
• Space Complexity: O(1)

C++

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        int n = nums.size();
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] <= 0 || nums[i] > n) {
                nums[i] = n + 1;
            }
        }       
        for (int i = 0; i < nums.size(); i++) {
            int temp = abs(nums[i]);
            if (temp > 0 && temp <= n) {
                nums[temp - 1] = -abs(nums[temp-1]);
            }
        }
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] > 0)
                return i + 1;
        }
        return n + 1;
    }
};