B3300 [USACO2011 Feb]Best Parenthesis 模拟

这是我今天遇到最奇怪的问题，希望有人帮我解释一下。。。

一开始我能得90分：

#include<iostream>
#include<cstdio>
#include<cmath>
#include<ctime>
#include<queue>
#include<algorithm>
#include<stack>
#include<cstring>
using namespace std;
#define duke(i,a,n) for(int i = a;i <= n;i++)
#define lv(i,a,n) for(int i = a;i >= n;i--)
#define clean(a) memset(a,0,sizeof(a))
typedef long long ll;
const int INF =  << ;
const ll mod = ;

typedef double db;
template <class T>
void read(T &x)
{
    char c;
    bool op = ;
    while(c = getchar(), c < '' || c > '')
        if(c == '-') op = ;
    x = c - '';
    while(c = getchar(), c >= '' && c <= '')
        x = x *  + c - '';
    if(op) x = -x;
}
template <class T>
void write(T x)
{
    if(x < ) putchar('-'), x = -x;
    if(x >= ) write(x / );
    putchar('' + x % );
}
ll n,s = ;
ll p[],x,ans = ;
ll num = ;
int main()
{
    read(n);
    duke(i,,n)
    {
        read(x);
        if(x == )
        {
            p[++num] = x;
        }
        else
        {
            s = ;
            while (p[num] != )
            {
                s += p[num];
                s = s % mod;
                num--;
            }
            if(s == )
            {
                p[num] = ;
            }
            else
            {
                p[num] = s *  % mod;
            }
        }
    }
    ll tot = ;
    duke(i,,num)
    {
//        cout<<p[i]<<endl;
        tot += p[i];
        tot %= mod;
    }
    write(tot);
    return ;
}

然而并不知道为什么。。。找了个题解，发现没啥区别，然后就改了一下num的顺序，结果变成80，最神奇的是之前错的那个点对了，然而另两个点错了。。。

#include<iostream>
#include<cstdio>
#include<cmath>
#include<ctime>
#include<queue>
#include<algorithm>
#include<stack>
#include<cstring>
using namespace std;
#define duke(i,a,n) for(int i = a;i <= n;i++)
#define lv(i,a,n) for(int i = a;i >= n;i--)
#define clean(a) memset(a,0,sizeof(a))
typedef long long ll;
const int INF =  << ;
const ll mod = ;
typedef double db;
template <class T>
void read(T &x)
{
    char c;
    bool op = ;
    while(c = getchar(), c < '' || c > '')
        if(c == '-') op = ;
    x = c - '';
    while(c = getchar(), c >= '' && c <= '')
        x = x *  + c - '';
    if(op) x = -x;
}
template <class T>
void write(T x)
{
    if(x < ) putchar('-'), x = -x;
    if(x >= ) write(x / );
    putchar('' + x % );
}
ll n,s = ;
ll p[],x,ans = ;
ll num = ;
int main()
{
//    freopen("1.in","r",stdin);
//    freopen("1.out","w",stdout);
    read(n);
    p[] = ;
    p[] = ;
    clean(p);
    duke(i,,n)
    {
        read(x);
        if(x == )
        {
            p[num++] = x;
        }
        if(x == )
        {
            s = ;
            while (p[num - ] != )
            {
                s += p[num - ];
                s = s % mod;
                num--;
            }
            /*if(i == n)
            {
                printf("%lld\n",s);
                cout<<num - 1<<endl;
            }*/
            if(s == )
            {
                p[num - ] = ;
            }
            else
            {
                p[num - ] = (s * ) % mod;
            }
            /*if(i == n)
            {
                printf("%lld\n",p[num - 1]);
                cout<<num - 1<<endl;
            }*/
        }
    }
    ll tot = ;
    duke(i,,num - )
    {
//        cout<<p[i]<<endl;
        tot += p[i];
        tot %= mod;
    }
    write(tot);
    return ;
}

蒙了，然后一点点de,到最后也没看出来。通过一些手段发现了是算和的时候出问题了，然而并不知道为什么。。。

#include<iostream>
#include<cstdio>
#include<cmath>
#include<ctime>
#include<queue>
#include<algorithm>
#include<stack>
#include<cstring>
using namespace std;
#define duke(i,a,n) for(int i = a;i <= n;i++)
#define lv(i,a,n) for(int i = a;i >= n;i--)
#define clean(a) memset(a,0,sizeof(a))
typedef long long ll;
const int INF =  << ;
const ll mod = ;
typedef double db;
template <class T>
void read(T &x)
{
    char c;
    bool op = ;
    while(c = getchar(), c < '' || c > '')
        if(c == '-') op = ;
    x = c - '';
    while(c = getchar(), c >= '' && c <= '')
        x = x *  + c - '';
    if(op) x = -x;
}
template <class T>
void write(T x)
{
    if(x < ) putchar('-'), x = -x;
    if(x >= ) write(x / );
    putchar('' + x % );
}
ll n,s = ;
ll p[],x,ans = ;
ll num = ;
int main()
{
//    freopen("1.in","r",stdin);
//    freopen("1.out","w",stdout);
    read(n);
    p[] = ;
    p[] = ;
    clean(p);
    duke(i,,n)
    {
        read(x);
        if(x == )
        {
            p[num] = x;
            num++;
        }
        if(x == )
        {
            s = ;
            while (p[num - ] != )
            {
                s += p[num - ];
                s = s % mod;
                num--;
            }
            if(s == )
            {
                p[num - ] = ;
            }
            else
            {
                p[num - ] = (s * ) % mod;
            }
        }
    }
    for(int i=;i<num;i++)
    {
//        cout<<p[i]<<endl;
        p[] += p[i];
        p[] = p[] % ;
    }
    cout<<p[];
    return ;
}
/*
6
0
0
1
1
0
1
( ( ) ) ( )
0 0 1 2 0 1
*/

就是把

ll tot = ;
    duke(i,,num - )
    {
//        cout<<p[i]<<endl;
        tot += p[i];
        tot %= mod;
    }
    write(tot);

换成

for(int i=;i<num;i++)
    {
//        cout<<p[i]<<endl;
        p[] += p[i];
        p[] = p[] % ;
    }
    cout<<p[];

就对了！！！震惊！！！为什么？求大佬解释。。。