Manacher（最长递减回文串）

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <algorithm>
#include <iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include <stdio.h>
#include <string.h>
using namespace std;
int a[100009] , b[200009];
int p[200009];


int num;


int main()
{
    int t ;
    scanf("%d" , &t);
    while(t--)
    {
        int l ;
        scanf("%d" , &l);
        for(int i = 0 ; i < l ; i++)
        {
            scanf("%d" , &a[i]);
        }
        int len = 0 ;
        b[len++] = -1;
        b[len++] = -1 ;
        for(int i = 0 ; i < l ; i++)
        {
            b[len++] = a[i];
            b[len++] = -1 ;
        }
        int mx = -1 , mid , ans = 0;
        for(int i = 1 ; i < len ; i++)
        {
            if(mx > i)
            {
                p[i] = min(p[2*mid-i] , mx - i);
            }
            else
            {
                p[i] = 1 ;
            }
            while(b[i-p[i]] == b[i+p[i]] &&  b[i - p[i] + 2] >= b[i - p[i]])//从中间往两边递减
            {
                p[i]++;
            }
            if(mx < p[i] + i)
            {
                mid = i ;
                mx = p[i] + i;
            }
            ans = max(ans , p[i] - 1);
        }
        printf("%d\n" ,ans);
    }

    return 0 ;
}