41. First Missing Positive

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題目敘述

Given an unsorted integer array nums. Return the smallest positive integer that is not present in nums.

You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.

Example 1

Input: nums = [1,2,0] Output: 3 Explanation: The numbers in the range [1,2] are all in the array.

Example 2

Input: nums = [3,4,-1,1] Output: 2 Explanation: 1 is in the array but 2 is missing.

Example 3

Input: nums = [7,8,9,11,12] Output: 1 Explanation: The smallest positive integer 1 is missing.

解題思路

Solution

import java.util.Arrays;

class Solution {
    public int firstMissingPositive(int[] nums) {
        Arrays.sort(nums);
        int tmp = 1;

        for(int i = 0; i < nums.length; i++){
            if(nums[i] == tmp){
                tmp++;
            }else if(nums[i] > tmp){
                return tmp;
            }
        }

        return tmp;
    }
}