题目链接：https://www.acwing.com/problem/content/description/801/
题目如下：

#include <iostream>
#include <unordered_map>
#include <limits.h>
using namespace std;
const int N=100010;
int arr[N];
int maxlen(int n);
int main(){
    int n;
    cin>>n;
    for(int i=0;i<n;i++) cin>>arr[i];
    
    //carl哥的滑动窗口
    cout<<maxlen(n)<<endl;
    
    return 0;
}
int maxlen(int n){
    int res=INT_MIN;
    int l=0;
    unordered_map<int,int> umap;
    
    for(int r=0;r<n;r++){
        umap[arr[r]]++;
        while(umap[arr[r]]>=2){
            umap[arr[l++]]--;//关键处
        }
        
        res=max(res,r-l+1);
    }
    
    if(res==INT_MIN) return 0;
    else return res;
}