递归1--二分查找

递归1--二分查找

一、心得

递归的时候注意返回值，返回的时候是返回到上一层 

二、代码及结果

/*
二分查找：
可写递推和递归
其实这两个都只要把递推式子写出来就好

查找的数大于mid，left=mid+1
小于mid，right=mid-1
边界条件，等于mid，输出即可 
*/ 
/*
心得：注意返回值，返回的时候是返回到上一层 
*/
#include <iostream>
using namespace std;
//二分查找递归，设置标签 
bool binarySearch1(int a[],int left,int right,int num){//初始left=0，right=length-1 
    bool flag=false; 
    int mid=(left+right)/2;
    //cout<<mid<<endl;
    //cout<<num<<" "<<a[mid]<<endl;
    if(left<=right){
        if(num==a[mid]) {
            flag=true;
        }
        else if(num>a[mid]){
            //cout<<"right:"<<a[mid+1]<<" "<<a[right]<<endl;
            flag=binarySearch1(a,mid+1,right,num);
            
        }
        else if(num<a[mid]){
            //cout<<"left:"<<a[left]<<" "<<a[mid-1]<<endl;
            flag=binarySearch1(a,left,mid-1,num);    
        }
    }
    return flag;
} 

//二分查找非递归，不设置标签 
/*
不设置返回值的时候，有栈可以存值 
*/
bool binarySearch2(int a[],int left,int right,int num){//初始left=0，right=length-1 
    
    int mid=(left+right)/2;
    //cout<<mid<<endl;
    //cout<<num<<" "<<a[mid]<<endl;
    if(left<=right){
        if(num==a[mid]) {
            return true;
        }
        else if(num>a[mid]){
            //cout<<"right:"<<a[mid+1]<<" "<<a[right]<<endl;
            binarySearch2(a,mid+1,right,num);
            
        }
        else if(num<a[mid]){
            //cout<<"left:"<<a[left]<<" "<<a[mid-1]<<endl;
            binarySearch2(a,left,mid-1,num);    
        }
    }
    else{
        return false;
    } 
} 


//二分查找递推
bool binarySearch3(int a[],int left,int right,int num){//初始left=0，right=length-1 
    while(left<=right){
        int mid=(left+right)/2;
        if(num==a[mid]) return true;
        else if(num>a[mid]) left=mid+1;
        else if(num<a[mid]) right=mid-1;
    } 
    return false;
} 

int main(){
    int a[11]={1,2,4,7,8,9,11,15,17,19,20};
    //bool b=binarySearch1(a,0,10,20);
    //bool b=binarySearch2(a,0,10,7);
    bool b=binarySearch3(a,0,10,20);
    cout<<b<<endl;
    return 0;
}