#Leetcode# 41. First Missing Positive

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

 

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.

代码：

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        int n = nums.size();
        int temp = n + 1;
        for(int i = 0; i < n; i ++) {
            if(nums[i] >= 0) {
                temp = i;
                break;
            }
        }
        
        if(temp == n + 1) return 1;
        else {
            map<int, int> mp;
            for(int i = temp; i < n; i ++)
                mp[nums[i]] ++;
            for(int i = 1; i <= nums[n - 1]; i ++) {
                if(mp[i] == 0) 
                    return i;
            }
            return nums[n - 1] + 1;
        }
    }
};