二分查找 模板

int bsearch(int l, int h, int k)//二分查找函数
{
    int i, mid;
    
    while(l<=h){
        mid = l+(h-l)/2;
        if(X[mid]>k)
            h = mid-1;
        else if(X[mid]<k)
            l = mid+1;
        else 
            break;
    }
    return mid;
}

int max_bsearch(int l, int h, int k)//求上界
{
    int i, mid, ans = -1;

    while(l<=h){
        mid = l+(h-l)/2;
        if(X[mid]>=k){
            h = mid-1;
            ans =X[mid];
        }
        else
            l = mid+1;
    }
    return ans;
}

int min_bsearch(int l, int h,int k)//求下界
{
    int i, mid, ans = -1;

    while(l<=h){
        mid = l+(h-l)/2;
        if(X[mid]<=k){
            l = mid+1;
            ans = X[mid];
        }
        else
            h = mid-1;
    }
    return ans;
}

void Quick_sort(int l, int h)//复习一下快排
{
    int i = l, j = h;
    int v = X[l];

    if(l>=h)
        return ;
    while(i<j){
        while(i<j && X[j]>=v)j--;
        X[i] = X[j];
        while(i<j && X[i]<=v)i++;
        X[j] = X[i];
    }
    X[i] = v;
    Quick_sort(l,i-1);
    Quick_sort(i+1,h);
}

附上一道练习题

代码在这

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#define ll long long

using namespace std;

int X[1000003];

void Quick_sort(int l, int h)//复习一下快排
{
    int i = l, j = h;
    int v = X[l];

    if(l>=h)
        return ;
    while(i<j){
        while(i<j && X[j]>=v)j--;
        X[i] = X[j];
        while(i<j && X[i]<=v)i++;
        X[j] = X[i];
    }
    X[i] = v;
    Quick_sort(l,i-1);
    Quick_sort(i+1,h);
}

int max_bsearch(int l, int h, int k)//求上界
{
    int i, mid, ans = -1;

    while(l<=h){
        mid = l+(h-l)/2;
        if(X[mid]>=k){
            h = mid-1;
            ans =X[mid];
        }
        else
            l = mid+1;
    }
    return ans;
}

int bsearch(int l, int h, int k)//二分查找函数
{
    int i, mid;
    
    while(l<=h){
        mid = l+(h-l)/2;
        if(X[mid]>k)
            h = mid-1;
        else if(X[mid]<k)
            l = mid+1;
        else 
            break;
    }
    return mid;
}

int min_bsearch(int l, int h,int k)//求下界
{
    int i, mid, ans = -1;

    while(l<=h){
        mid = l+(h-l)/2;
        if(X[mid]<=k){
            l = mid+1;
            ans = X[mid];
        }
        else
            h = mid-1;
    }
    return ans;
}

int main()
{
    int n, m, i, j, k, a, b;

    while(scanf("%d %d",&n,&m)==2){
        for(i=0;i<n;i++)
            scanf("%d",X+i);
        Quick_sort(0,n-1);
        for(i=0;i<m;i++)
        {
            scanf("%d",&k);
            a = min_bsearch(0,n-1,k);
            b = max_bsearch(0,n-1,k);
            if(a == -1)// 1
                printf("%d
",b);
            else if(b == -1)// 2
                printf("%d
",a);
            else if(a==b)
                printf("%d
",a);
            else if(k-a == b-k)
                printf("%d %d
",a,b);
            else if(k-a>b-k)
                printf("%d
",b);
            else
                printf("%d
",a);
        }
        //1和2两条语句是非常重要的不然的话可以测试一下 n=1 ,m=5,X[0]=1,k=0这组数据。
        printf("
");
    }
    return 0;
}

当然你可以调用C中的<stdlib.h>中的qsort或C++中的<algorithm>sort。二分查找就用C中的<stdlib.h>中的bsearch。