【PAT甲级 - C++题解】1144 The Missing Number

1144 The Missing Number

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 (≤105). 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

题意

给定一个数组，找到其中未出现的最小正整数。

思路

我们可以将所有数存进一个 set 容器，它会自动帮我们去掉重复的数字并排序，然后再从 1 往后查找，第一个没有在 set 中找到的数字就是我们要找的最小正整数。

代码

#include<bits/stdc++.h>
using namespace std;
const int N = 100000;
unordered_set<int> res;
int main()
{
    int n;
    cin >> n;
    //将正数放入容器
    for (int i = 0; i < n; i++)
    {
        int x;
        cin >> x;
        res.insert(x);
    }
    //找到最小未出现的正整数
    int t;
    for (t = 1; t <= N; t++)
    {
        if (res.count(t) == 0)
        {
            cout << t << endl;
            return 0;
        }
    }
    cout << t << endl;
    return 0;
}