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41. First Missing Positive 👍

  • Time: $O(n)$
  • Space: $O(1)$
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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;
  }
};

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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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class Solution:
  def firstMissingPositive(self, nums: list[int]) -> int:
    n = len(nums)

    # Correct slot:
    # nums[i] = i + 1
    # nums[i] - 1 = i
    # nums[nums[i] - 1] = nums[i]
    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, num in enumerate(nums):
      if num != i + 1:
        return i + 1

    return n + 1