二分查找 Binary Search

left = mid + 1不能写为left = mid,right同理。否则可能陷入无限循环。不论right初始化为size还是size-1，while语句必须是left <= right。否则即使有也有可能找不到值。

#include <stdio.h>

#define ARRAY_SIZE 7 /* size of array */

int main(void)
{
    int a[ARRAY_SIZE] = {1,2,3,4,5,6,7}; /* sorted array */
    int left = 0; /* start key of index */
    int right = ARRAY_SIZE; /* end key of index */
    int mid; /* middle key of index */
    int value; /* search value */

    puts("Find value?");
    scanf("%d", &value);

    while(left <= right) {
        mid = (left + right) / 2; /* calc of middle key */
        if (a[mid] == value) {
            puts("Found!\n");
            return 0;
        } else if (a[mid] < value) {
            left = mid + 1; /* adjustment of left(start) key */
        } else {
            right = mid - 1; /* adjustment of right(end) key */
        }
    }
    puts("Not Found.\n");
    return 0;
}