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。。。