大佬求助，思路感觉没毛病，代码哪错了？急！

#include<iostream>
#include<algorithm>
using namespace std;
int main(){
	int a_len;
	cin>>a_len;
	int a[a_len],swap[a_len],sum=0;
	for(int i = 0;i <= a_len-1;i ++){
		cin>>a[i];
	}
	sort(a,a+a_len);
	for(int i = 0;i <= a_len-1;i ++){
		if(a[i+1] != a[i] + 1){
			cout<<a[i] + 1;
			break;
		}
	}
}