/* 二分查找/折中查找
 * binsearch(int 待查找数，int 被查找数组，int 数组长度) 
 * 查找成功返回索引值，失败返回-1
 * 限制：数组必须是"升序"的整型数组
 */ 
int binsearch(int x, int v[], int n){
    int low, high, mid;
    low = 0;
    high = n - 1;
    while(low <= high){
        mid = (low + high) / 2;
        if(x > v[mid]){
            low = mid + 1;
        }else if(x < v[mid]){
            high = mid - 1;
        }else{
            return mid;
        }
    }
    return -1;
}