[刷题] 二分查找

浙大mooc习题：

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

#define MAXSIZE 10
#define NotFound 0
typedef int ElementType;

typedef int Position;
typedef struct LNode *List;
Position BinarySearch( List L, ElementType X );

struct LNode {
    ElementType Data[MAXSIZE];
    Position Last; /* 保存线性表中最后一个元素的位置 */
};

List ReadInput(); /* 裁判实现，细节不表。元素从下标1开始存储 */
Position BinarySearch( List L, ElementType X );

int main()
{
    List L;
    ElementType X;
    Position P;

    L = ReadInput();
    scanf("%d", &X);
    P = BinarySearch( L, X );
    printf("%d
", P);

    return 0;
}
Position BinarySearch( List L, ElementType X ){
    int p = 0;
    int l = 1;
    int r =  L->Last;
    int num = r/2;
    while(num--){
        int mid = (l + r)/2;
        if(L->Data[mid] == X){
            p = mid;
            break;
        }
        else if(L->Data[r] == X){
            p = r;
            break;
        }
        else if(L->Data[mid] > X)
            r = mid;
        else if(L->Data[mid] < X)
            l = mid;
    }
    if(p == 0)
        return NotFound;
    return p;
}