First Missing Positive

Problem: First Missing Positive

We can mark numbers at position with appearing indexes as negative. For original negative numbers, we modify them as a number bigger then length of array at first. If there remains positive numbers, the index of that position is missing.

Code in Python:

class Solution(object):
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        n = len(nums)
        for i in xrange(n):
            if nums[i] <= 0: nums[i] = len(nums)+1
        for i in xrange(n):
            if abs(nums[i]) <= n: nums[abs(nums[i])-1] = -abs(nums[abs(nums[i])-1])
        for i in xrange(n):
            if nums[i] > 0: return i+1
        return n+1

Code in Java:

public class Solution {
    public int firstMissingPositive(int[] nums) {
        if (nums.length == 0 || nums == null) return 1;

        for (int i = 0; i < nums.length; i++) {
            if (nums[i] <= 0) nums[i] = nums.length+1;
        }
        for (int i = 0; i < nums.length; i++) {
            if (Math.abs(nums[i]) <= nums.length) nums[Math.abs(nums[i])-1] = -Math.abs(nums[Math.abs(nums[i])-1]);
        }
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0) return i+1;
        }
        return nums.length+1;
    }
}