二分查找

二分查找又称折半查找；
要求：
1.必须采用顺序存储结构；
2.必须按关键字大小有序排列；
优点：

• 比较次数少，查找速度快，平均性能好；

缺点：

• 要求待查表为有序表，且插入删除困难；-

二分查找算法就是不断将数组进行对半分割，每次拿中间元素和要查找的数进行比较；

#include <iostream>using namespace std;//二分查找(非递归版本)int search(int* a, int len, int goal){    int left = 0;    int right = len - 1;    while(left <= right)    {        int mid = (left + right)>>1;//位运算节省时间        if(a[mid] == goal)            return mid;        else if(a[mid] > goal)//在左半边            high = mid - 1;        else                    //在右半边            low = mid + 1;    }    //没找到    return -1;}//递归版本int Search(int *a,int left,int right,int goal){    if (left<=right)    {        int mid = (left+right)>>1;        if(goal == a[mid])            return mid;        else if(goal<a[mid])            return Search(a,left,mid-1,goal);        else if(goal>a[mid])            return Search(a,mid+1,right,goal);    }    else        return -1;}int main(){    const int LEN  = 10000;    int a[LEN];    for(int i = 0; i < LEN; i++)        a[i] = i - 5000;    int goal = 0;    int index1 = search(a, LEN, goal);    if(index1 != -1)        cout<<goal<<"在数组中的下标为"<<search(a, LEN, goal)<<endl;    else        cout<<"不存在"<<goal<<endl;    int index2= Search(a, 0, len-1,len, goal);    if(index2 != -1)        cout<<goal<<"在数组中的下标为"<<Search(a, 0, len-1,len, goal)<<endl;    else        cout<<"不存在"<<goal<<endl;    return 0;}