LeetCode First Missing Positive Map

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.

Solution

我们只需要求出最小的缺失正数，找到一个下界即可：用 $Map$ 记录原数组，然后从 $1$ 遍历原数组个数的连续正数即可。

点击查看代码

class Solution {
private:
    map<int,int> mp;
public:
    int firstMissingPositive(vector<int>& nums) {
        int ans = 1;
        int n = nums.size();
        
        if(n==1){
            if(nums[0]<1)return 1;
            else if(nums[0]==1)return 2;
            else return 1;
        }
        int fg = 0;
        for(int i=0;i<n;i++){
            if(nums[i]<0)continue;
            else{
                mp[nums[i]]=1;
            }
        }
        for(int i=0;i<n;i++){
            if(!mp[ans])return ans;
            else ans++;
        }
        return ans;
    }
};