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First Missing Positive Integer

First Missing Positive Integer

Given an array of integers, find the smallest missing positive integer.

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Statement

Given an unsorted integer array, nums, find the smallest positive integer that is missing from the array.

Implement a solution that takes O(n)O(n) time and constant space.

Examples

Let’s look at some arrays and the first missing positive integer in each:

Sample input

[1, 9, 14, 11, 7, 13]

Expected output

2

Try it yourself

#include <iostream>
#include <vector>
using namespace std;
int FirstMissingPositive(vector<int> nums) {
	// Write your code here
	return -1;
}

Solution

First, we check the base case. To verify that the first missing positive integer is not 1, we check for its presence in the array. We use the array below as an example. Since 1 exists in the array, it’s not our missing positive integer:

g array -1 2 0 37 4 1 3 -4 6 21
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