First Missing Positive LeetCode Solution | Easy Approach

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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 1:

Input: nums = [1,2,0]
Output: 3

Example 2:

Input: nums = [3,4,-1,1]
Output: 2

Example 3:

Input: nums = [7,8,9,11,12]
Output: 1

Constraints:

  • 1 <= nums.length <= 5 * 105
  • -231 <= nums[i] <= 231 - 1

First Missing Positive Solutions

Time: O(n)
Space: O(1)

C++

class Solution {
 public:
  int firstMissingPositive(vector<int>& nums) {
    const int n = nums.size();

    // Correct slot:
    // nums[i] = i + 1
    // nums[i] - 1 = i
    // nums[nums[i] - 1] = nums[i]
    for (int i = 0; i < n; ++i)
      while (nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i] - 1])
        swap(nums[i], nums[nums[i] - 1]);

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

    return n + 1;
  }
};

Java

 class Solution {
  public int firstMissingPositive(int[] nums) {
    final int n = nums.length;

    // Correct slot:
    // nums[i] = i + 1
    // nums[i] - 1 = i
    // nums[nums[i] - 1] = nums[i]
    for (int i = 0; i < n; ++i)
      while (nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i] - 1])
        swap(nums, i, nums[i] - 1);

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

    return n + 1;
  }

  private void swap(int[] nums, int i, int j) {
    final int temp = nums[i];
    nums[i] = nums[j];
    nums[j] = temp;
  }
}

Python

class Solution {
  public int firstMissingPositive(int[] nums) {
    final int n = nums.length;

    // Correct slot:
    // nums[i] = i + 1
    // nums[i] - 1 = i
    // nums[nums[i] - 1] = nums[i]
    for (int i = 0; i < n; ++i)
      while (nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i] - 1])
        swap(nums, i, nums[i] - 1);

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

    return n + 1;
  }

  private void swap(int[] nums, int i, int j) {
    final int temp = nums[i];
    nums[i] = nums[j];
    nums[j] = temp;
  }
}

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