readme.md

41. First Missing Positive

Hard

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

Solution

public class Solution {
    public int FirstMissingPositive(int[] nums) {
        for (int i = 0; i < nums.Length; i++) {
            if (nums[i] <= 0 || nums[i] > nums.Length || nums[i] == i + 1) {
                continue;
            }
            Dfs(nums, nums[i]);
        }
        for (int i = 0; i < nums.Length; i++) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return nums.Length + 1;
    }

    private void Dfs(int[] nums, int val) {
        if (val <= 0 || val > nums.Length || val == nums[val - 1]) {
            return;
        }
        int temp = nums[val - 1];
        nums[val - 1] = val;
        Dfs(nums, temp);
    }
}