二分查找 有序数组
int mid = (low + high)/2;
if (a [mid] == key) {
Console.WriteLine ("find key,index", mid);
return mid; //返回值 mid
} else if (a[mid] > key) {
high = mid - 1;
} else if (a[mid] < key) {
low = mid + 1;
}
int[] arr = {1,2,3,4,5,6,7,8 };
int low = 0, hight = arr.Length - 1; //定义第一位和最后一个的下标
int key = 7;
while (low <= hight) {
int mid = (low + hight) / 2;
if (arr[mid] == key) {
Console.WriteLine ("find,index="+mid); //找到，下标为 ：
break;
}else if (arr[mid]>key) {
hight = mid - 1;
}else if (arr[mid]<key) {
low = mid + 1;
}
}
if (low > hight){
Console.WriteLine ("404");
}
