双指针+回文数组

https://codeforces.com/problemset/problem/1610/B

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

#define endl '\n'
using ll = long long;
using pii = pair<int, int>;
const double PI = acos(-1);
const int N = 2e5+10;
const int mod = 1e9 + 7;
int a[N];
void solve() {
	int n;cin>>n;
	for(int i=1;i<=n;i++)
		cin>>a[i];
		int l=1,r=n;
		int x=0,y=0;
		bool flag=0;
		while(l<r){
			if(a[l]==a[r]){
				l++;
				r--;
			}
			else{
				x=a[l];
				y=a[r];
				flag=1;
				break;
			}
		}
		if(!flag){
			cout<<"YES"<<endl;
			return;
		}
		l=1,r=n;
		bool flag1=0,flag2=0;
		while(l<r){
			if(a[l]==a[r]){
				l++;
				r--;
			}
			else{
				if(a[l]==x) l++;
				else if(a[r]==x) r--;
				else {
					flag1=1;
					break;
				}
			}
		}
		l=1,r=n;
		while(l<r){
			if(a[l]==a[r]){
				l++;
				r--;
			}
			else{
				if(a[l]==y) l++;
				else if(a[r]==y) r--;
				else {
					flag2=1;
					break;
				}
			}
		}
		if(flag1==1&&flag2==1){
			cout<<"NO"<<endl;
		}
		else cout<<"YES"<<endl;
}


int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr), cout.tie(nullptr);
	
	int T = 1;
	cin>>T;
	while (T--) {
		solve();
	}
	
	return 0;
}