41. First Missing Positive

Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

Similar: 287. Find the Duplicate Number

268. Missing Number

public class Solution {
    public int firstMissingPositive(int[] nums) {
        if (nums.length == 0) return 1;
        
        int n = nums.length;
        int i = 0;
        while (i < n) {
            if (nums[i] > 0 && nums[i] < n && nums[i] != i && nums[nums[i]] != nums[i]) { // in case duplicate as [1,1]
                int tmp = nums[nums[i]];
                nums[nums[i]] = nums[i];
                nums[i] = tmp;
            } else {
                i++;
            }
        }
        
        i = 1;
        while(i < n) {
            if (nums[i] != i) {
                return i;
            }
            i++;
        }
        return nums[0] == n ? n+1 : n;
    }
}