二分查找

二分查找

二分法查找

  1 #include <stdio.h>
  2 #include <stdlib.h>
  3 #define N 100
  4 int compare(int middle,int key)
  5 {
  6     if(0 > middle -key){
  7     return -1;
  8     }
  9     else if(0 < middle - key){
 10         return 1;
 11     }
 12     else {
 13             return 0;
 14     }
 15 }
 16 int binarySearch(const int list[],int key,int length)
 17 {
 18         int left=0,right=length-1,middle;
 19         while(left <= right){
 20         middle = (right + left)/2 ;
 21         switch ( compare(list[middle],key) ){
 22         case -1: left = middle+1; break;
 23         case 1: right=middle-1; break;
 24         case 0:return middle;
 25         }
 26         }
 27         return -1;
 28 }
 29 
 30 int main(void)
 31 {
 32         int a[N]={1,2,3,4,5,6,7,8,9};
 33         printf("%d
",a[binarySearch(a,2,9)]);
 34         return 0;
 35 }