折半查找（二分查找）

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

//二分查找
typedef int ElemType;
typedef struct {
    ElemType * elem;
    int TableLen;
}SSTable;

void ST_Init(SSTable &ST,int len){
    ST.TableLen = len;
    ST.elem=(ElemType*) malloc(sizeof(ElemType)*ST.TableLen);
    int i;
    srand(time(NULL));
    for(i=0;i<ST.TableLen;i++){
        ST.elem[i]=rand()%100;
    }
}

void ST_Print(SSTable ST){
    for (int i = 0; i < ST.TableLen; i++) {
        printf("%3d",ST.elem[i]);
    }
    printf("\n");
}

int BinarySearch(SSTable ST,ElemType key){
    int low=0;
    int mid;
    int high=ST.TableLen-1;
    while(low<=high){//low<=high,可以让mid既能取到low,也能取到high
        mid=(low+high)/2;
        if(key>ST.elem[mid]){
            low=mid+1;
        }
        else if(key<ST.elem[mid]){
            high=mid-1;
        }
        else{
            return mid;
        }
    }
    return -1;
}

//函数名中存储的是函数的入口地址，也是一个指针，是函数指针类型
//left指针和right指针是指向数组中的任意两个元素
//qsort规定如果left指针指向的值大于right指针指向的值，返回正值，小于返回负值，相等返回0
int compare(const void *left,const void *right){
   return *(int*)left-*(int*)right;
}

int main() {
   SSTable ST;
   ST_Init(ST,10);
   ST_Print(ST);
    qsort(ST.elem,ST.TableLen,sizeof (ElemType),compare);
    ST_Print(ST);
   ElemType key;
    printf("please input search key:\n");
    scanf("%d",&key);
    int pos = BinarySearch(ST,key);
    if(pos!=-1){
        printf("sucess search,it's NO.%d\n",pos+1);
    }
    else{
        printf("false search\n");
    }
    return 0;
}