41. First Missing Positive

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LeetCode

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Problem

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:

• $1 <= nums.length <= 10^5$
• $-2^{31} <= nums[i] <= 2^{31} - 1$

Code

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

        for(int i = 0; i < nums.length; i++){
            // 交换数字直到位置正确
            while(nums[i] > 0 && nums[i] <= nums.length && nums[nums[i] - 1] != nums[i]){
                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;
    }
}