1 int gcd(int n,int m)//n>m
 2 {
 3     //最大公约数
 4     int r;
 5     while(m)
 6     {
 7         r = n%m;
 8         n = m;
 9         m = r;
10     }
11     return n;
12 }
13 
14 int kgcd(int a,int b)
15 {
16     if(!a) return b;
17     if(!b) return a;
18     if(!(a&1) && !(b&1))
19         return kgcd(a>>1,b>>1)<<1;
20     else if(!(b&1)) return kgcd(a,b>>1);
21     else if(!(a&1)) return kgcd(a>>1,b);
22     else return kgcd(abs(a-b),min(a,b));
23 }
24 
25 //扩展欧几里得
26 //求方程ax+by+c = 0有多少整数解
27 int extgcd(int a,int b,int &x,int &y)
28 {
29     if(!b)
30     {
31         x=1;
32         y=0;
33         return a;
34     }
35     int d = extgcd(b,a%b,x,y);
36     int t = x;
37     x=y;
38     y=t-a/b*y;
39     return d;
40 }