51nod 1423 最大二“货” 单调栈

#include<stdio.h>
#include<math.h>
#include<cstring>
#include<stack>
#include<iostream>
#include<algorithm>
#include<queue>
#define MAXSIZE 100005
#define LL long long

using namespace std;
const LL INF=1e19;

LL a[MAXSIZE];
int s[MAXSIZE];

int main()
{
    int n,top;
    LL maxn = 0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lld",&a[i]);
    a[0] = a[n+1] = 0;
    top = 0;
    for(int i=1;i<=n+1;i++)
    {
        if(top==0)
        {
            s[++top] = i;
        }

        else
        {
            while(top>=1 && a[s[top]] < a[i])
            {
                maxn = max(maxn,a[i]^a[s[top]]);
                top--;
            }
            maxn = max(maxn,a[i]^a[s[top]]);
            s[++top] = i;
        }
    }

    for(int i=n;i>=0;i--)
    {
        if(top==0)
        {
            s[++top] = i;
        }

        else
        {
            while(top>=1 && a[s[top]] < a[i])
            {
                maxn = max(maxn,a[i]^a[s[top]]);
                top--;
            }
            maxn = max(maxn,a[i]^a[s[top]]);
            s[++top] = i;
        }
    }
    printf("%lld\n",maxn);
    return 0;
}