【PAT甲级】The Missing Number

Problem Description：

Given N integers, you are supposed to find the smallest positive integer that is NOT in the given list.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (≤10​5​​). Then N integers are given in the next line, separated by spaces. All the numbers are in the range of int.

Output Specification:

Print in a line the smallest positive integer that is missing from the input list.

Sample Input:

10
5 -25 9 6 1 3 4 2 5 17

Sample Output:

7

解题思路：

肯定是先要无脑set来记录所有出现在输入列表中的正整数啊，然后再无脑for循环找出第一个（也就是最小的那个）没有出现在set中的正整数即可。

AC代码： 

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int N;
    cin >> N;
    set<int> s;  //用来存放出现过的正整数
    for(int i = 0; i < N; i++)
    {
        int temp;
        cin >> temp;
        if(temp > 0)  //向set中插入正整数
        {
            s.insert(temp);
        }
    }
    int ans;   //第一个没出现过的正整数ans
    for(int i = 1; ; i++)
    {
        if(s.count(i) == 0)  //找到第一个没出现过的正整数
        {
            ans = i;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}