二分查找的应用

递增序列的查找
补充：return -1的解释

#include <iostream>
using namespace std;
int s[1000];
int search(int str[],int left,int right,int num){
    while(left<=right){//要不断循环查找-while:既可判断，又可循环
        int mid=(left+right)/2;
        if(num==str[mid])
        return mid;
        else if(num<str[mid])
        right=mid-1;//在左边找
        else left=mid+1;//在右边找
    }
     return -1;//非法区间
}
int main(){
    int n,num;
    cin>>n>>num;
    for(int i=0;i<n;i++)
    cin>>s[i];
    cout<<search(s,0,n-1,num)<<endl;
    return 0;
}

同理递减序列的查找

#include <iostream>
using namespace std;
int s[1000];
int search(int str[],int left,int right,int num){
    while(left<=right){//要不断循环查找-while:既可判断，又可循环
        int mid=left+(right-left)/2;//等同(left+right)/2,可避免因left与right过大而溢出的问题
        if(num==str[mid])
        return mid;
        else if(num<str[mid])
        left=mid+1;//在右边找
        else right=mid-1;//在左边找
    }
     return -1;//非法区间
}
int main(){
    int n,num;
    cin>>n>>num;
    for(int i=0;i<n;i++)
    cin>>s[i];
    cout<<search(s,0,n-1,num)<<endl;
    return 0;
}

如何计算根号数的近似值

#include <cstdio>
#include<cmath>//sqrt
using namespace std;
const double eps=1e-5;
double Func(double n){//求平方的函数，求平方根
    return n*n;
}
double index(double left,double right){//区间
    double mid=0.0;
    while(left-right>eps){
        mid=(left+right)/2.0;
        if(Func(mid)>right)//以mid为准，近似数在左端
        right=mid;
        else left=mid;
    }
    return mid;//相当于left与right
}
int main(){
    double l,r;
    scanf("%lf%lf",&l,&r);
    double y=index(l,r);
    printf("%.6lf",y);
    return 0;
}