【排序】

1. 归并排序＜可求逆序对＞

   #include<iostream>
   #include<cstdio>
   using namespace std;
   long long n, a[1000100], c[1000100];
   long long ans=0;
   void merge(int l, int r)
   {
       if(l==r)    return ;
       int mid = (l + r) / 2;
       merge(l,mid);
       merge(mid+1,r);
       int i = l, j = mid + 1, k = 1;
       while (i <= mid && j <= r)
       {
           if (a[i] <= a[j])
               c[k++] = a[i++];
           else
           {    c[k++] = a[j++];
               ans+=mid-i+1;
           }
       }
       while (i <= mid)    c[k++] = a[i++];
       while (j <= r)    c[k++] = a[j++];    
       for (i = l, k = 1; i <= r; i++)
           a[i] = c[k++];
       
   }
   int main()
   {
       cin >> n;
       for (int i = 1; i <= n; i++)
           cin >> a[i];
       merge(1,n);
       cout<<ans<<endl;
       return 0;
   }

2. 快速幂

   #include<iostream>
   using namespace std;
   //求b^P
   int b,p;
   void work()
   {
       long long ans=1;
       int t=p,s=b;
       while(t!=0)
       {
           if(t%2==1)  ans=ans*s;
           t/=2;
           s=s*s;
       }
   }
   int main()
   {
       cin>>b>>p;
       work();
       cout<<ans<<endl;
       return 0;
   }

3. 二分

   #include<iostream>
   using namespace std;
   int a[10010],x;
   int find(int x)
   {
       int l=1,r=n;
       while(l+1<r)
       {
           int mid=(l+r)/2;
           if(a[mid]==x)   return mid;
           if(a[mid]>x)    r=mid;
           if(a[mid]<x)    l=mid;
       }
       if(a[l]==x) return l;
       if(a[r]==x) return r;
       return -1;
   }
   int main()
   {
       cin>>x;
       cout<<find(x)<<endl;
       return 0;
   }

4. quicksort

   /*
   Language:c++;
   By Kaike;
   Date:2016.09.25;
   */
   #include<iostream>
   using namespace std;
   int n,a[10010];
   void quicksort(int l,int r)
   {
       if(l>r) return ;
       int t=a[l];
       int i=l,j=r;
       while(i!=j)
       {
           while(a[j]>=temp && i<j)
               j--;
           while(a[i]<=temp && i<j)
               i++;
           if(i<j)
               swap(a[i],a[j]);
       }
       a[l]=a[i];
       a[i]=temp;
       quicksort(l,i-1);
       wuicksort(i+1,right);
       return ;
   }
   int main()
   {
       cin>>n;
       for(int i=1;i<=n;i++)
           cin>>a[i];
       quicksort(1,n);
       for(int i=1;i<=n;i++)
           cout<<a[i];
       return 0;
   }