41. 缺失的第一个正数 First Missing Positive

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

Follow up: Could you implement an algorithm that runs in O(n) time and uses constant extra space.?

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

 

方法：

将小于0和大于n的值置为n+1

将绝对值在1到n之间的数，标记i-1位置为负

第一个正数+1则是确实值

 

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

 

参考链接：

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

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