Description
某城市的街道呈网格状,左下角坐标为A(0, 0),右上角坐标为B(n, m),其中n >= m。现在从A(0, 0)点出发,只能沿着街道向正右方或者正上方行走,且不能经过图示中直线左上方的点,即任何途径的点(x, y)都要满足x >= y,请问在这些前提下,到达B(n, m)有多少种走法。
Input
输入文件中仅有一行,包含两个整数n和m,表示城市街区的规模。
Output
输出文件中仅有一个整数和一个换行/回车符,表示不同的方案总数。
Sample Input
6 6
Sample Output
132
HINT
100%的数据中,1 <= m <= n <= 5 000
Source
Solution
这题n==m时是组合数
然后正解是类似于catalan数的证明方法C(n+m,n)-C(n+m,n+1)
发一发高精度的板子QAQ
#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int mt=10000+5;
struct bign
{
int len,s[mt];
bign ()
{
memset(s, 0, sizeof(s)),len = 1;
}
bign (int num) { *this = num; }
bign (const char *num) { *this = num; }
bign operator = (const int num)
{
char s[mt];
sprintf(s, "%d", num);
*this = s;
return *this;
}
bign operator = (const char *num)
{
for(int i = 0; num[i] == '0'; num++) ;
len = strlen(num);
for(int i = 0; i < len; i++) s[i] = num[len-i-1] - '0';
return *this;
}
bign operator + (const bign &b) const
{
bign c;
c.len = 0;
for(int i = 0, g = 0; g || i < max(len, b.len); i++)
{
int x = g;
if(i < len) x += s[i];
if(i < b.len) x += b.s[i];
c.s[c.len++] = x % 10;
g = x / 10;
}
return c;
}
void clean()
{
while(len > 1 && !s[len-1]) len--;
}
bign operator * (const bign &b)
{
bign c;
c.len = len + b.len;
for(int i = 0; i < len; i++)
{
for(int j = 0; j < b.len; j++)
{
c.s[i+j] += s[i] * b.s[j];
}
}
for(int i = 0; i < c.len; i++)
{
c.s[i+1] += c.s[i]/10;
c.s[i] %= 10;
}
c.clean();
return c;
}
bign operator - (const bign &b)
{
bign c;
c.len = 0;
for(int i = 0, g = 0; i < len; i++)
{
int x = s[i] - g;
if(i < b.len) x -= b.s[i];
if(x >= 0) g = 0;
else
{
g = 1;
x += 10;
}
c.s[c.len++] = x;
}
c.clean();
return c;
}
bign operator / (const bign &b)
{
bign c, f = 0;
for(int i = len-1; i >= 0; i--)
{
f = f*10;
f.s[0] = s[i];
while(f >= b)
{
f =f-b;
c.s[i]++;
}
}
c.len = len;
c.clean();
return c;
}
bool operator < (const bign &b)
{
if(len != b.len) return len < b.len;
for(int i = len-1; i >= 0; i--)
{
if(s[i] != b.s[i]) return s[i] < b.s[i];
}
return false;
}
bool operator > (const bign &b)
{
if(len != b.len) return len > b.len;
for(int i = len-1; i >= 0; i--)
{
if(s[i] != b.s[i]) return s[i] > b.s[i];
}
return false;
}
bool operator == (const bign &b)
{
return !(*this > b) && !(*this < b);
}
bool operator != (const bign &b)
{
return !(*this == b);
}
bool operator <= (const bign &b)
{
return *this < b || *this == b;
}
bool operator >= (const bign &b)
{
return *this > b || *this == b;
}
string str() const
{
string res = "";
for(int i = 0; i < len; i++) res = char(s[i]+'0') + res;
return res;
}
};
istream& operator >> (istream &in, bign &x)
{
string s;
in >> s;
x = s.c_str();
return in;
}
ostream& operator << (ostream &out, const bign &x)
{
out << x.str();
return out;
}
int main()
{
bign a,b,d=1,e;
cin>>a>>b;
bign c=a+b,y=1;
for(bign i=a+y;i<=c;i=i+y) d=d*i;
for(bign i=y;i<=b;i=i+y) d=d/i;
e=d*b/(a+y);
bign ans=d-e;
ans.clean();
cout<<ans<<endl;
return 0;
}
/**************************************************************
Problem: 3907
User: ictsing
Language: Python
Result: Accepted
Time:34 ms
Memory:8200 kb
****************************************************************/
fac={};
def C(n,m):
return fac[n]/fac[m]/fac[n-m];
fac[0]=1;
for i in range(1,10000):
fac[i]=fac[i-1]*i;
f=raw_input().split(" ");
n=int(f[0]);m=int(f[1]);
print(C(n+m,n)-C(n+m,n+1));
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