## 二分查找算法
```java
/**
* 二分查找,前提是數組有序
* @param arr int[]
* @param key int
* @return int
*/
private static int binarySearch(int[] arr, int key) {
if (arr.length == 1) {
return 0;
}
if (!exist(arr, key)) {
return -1;
}
int low = 0;
int high = arr.length - 1;
while (low <= high) {
int mid = (low + high) / 2;
if (key < arr[mid]) {
high = mid - 1;
} else if (key > arr[mid]) {
low = mid + 1;
} else {
return mid;
}
}
return -(low + 1);
}
```
