41. First Missing Positive (Hard)

https://leetcode.com/problems/first-missing-positive/

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

Example 1:

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

Example 2:

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

Example 3:

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

Note:

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

Hints

Solutions

  • Java
class Solution {

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

        for (int i = 0; i < nums.length; i++) {
            // Swap the number until they are in the right position.
            while (nums[i] >= 1 && nums[i] <= nums.length && nums[i] != nums[nums[i] - 1]) {
                int temp = nums[nums[i] - 1];
                nums[nums[i] - 1] = nums[i];
                nums[i] = temp;
            }
        }

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