41. First Missing Positive

Posted 2020-11-16 lychnis

41. First Missing Positive

Hard

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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.

 

 

感觉这题像是为了答案量身打造的一样.这种贪心算法是怎么想出来的

把每个正整数放到对应的索引上即可... 

核心思想就是把正整数尽量往索引上放,没有被放的数字一定是特别大的或者是负数,那么这个索引再加上1就是我们要的答案

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        for(int i=0;i<nums.size();++i)
        {
            while(nums[i]>0&&nums[i]<nums.size()&&nums[nums[i]-1]!=nums[i])
                swap(nums[i],nums[nums[i]-1]);
        }
        for(int i=0;i<nums.size();++i)
            if(nums[i]!=i+1)
                return i+1;
        return nums.size()+1;
    }
};