分治基本算法

//n个数m个询问小于等于x的数 
#include<iostream>
#include<cstdio>
using namespace std;
int a[100];
int n,m;
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++)
    scanf("%d",&a[i]);
    a[0]=-1;
    for(int i=1;i<=m;i++)
    {
        int x,left=1,right=n,mid;
        scanf("%d",&x);
        while(left<=right)
        {
            mid=(left+right)/2;
            if(a[mid]<=x)left=mid+1;
            else right=mid-1;
        }
        printf("%d\n",a[right]);
    }
    return 0;
}

//快排 
#include<iostream>
#include<cstdio>
using namespace std;
int n,a[100];
void qsort(int l,int r)
{
    int i=l,j=r,mid=a[(l+r)/2];
    while(i<=j)
    {
        while(a[i]<mid)i++;
        while(a[j]>mid)j--;
        if(i<=j)swap(a[i],a[j]),i++,j--;
    }
    if(l<j)qsort(l,j);
    if(r>i)qsort(i,r);
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    scanf("%d",&a[i]);
    qsort(1,n);
    for(int i=1;i<=n;i++)
    printf("%d ",a[i]);
    return 0;
 } 

//归并排序求逆序对 
#include<iostream>
#include<cstdio>
using namespace std;
int n,tot,a[100],temp[100];
void merge_sort(int left,int right)
{
    if(left==right)return;
    int mid=(left+right)/2;
    merge_sort(left,mid);
    merge_sort(mid+1,right);
    int p=left,i=left,j=mid+1;
    while(i<=mid&&j<=right)
    {
        if(a[i]>a[j])
        {
            tot=tot+mid-i+1;temp[p++]=a[j++];
        }
        else
        temp[p++]=a[i++];
    }
    while(i<=mid)temp[p++]=a[i++];
    while(j<=right)temp[p++]=a[j++];
    for(int i=left;i<=right;i++)a[i]=temp[i]; 
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    scanf("%d",&a[i]);
    merge_sort(1,n);
    cout<<tot<<endl;
    for(int i=1;i<=n;i++)
    printf("%d ",a[i]);
    return 0;
}