二分查找

public static int binarySearch(int[] arr,int target)
{
        int low = 0;
        int high = arr.length - 1;
        int mid=-1;
        
        while (low <= high)
        {
            mid = (high-low)/2 + low ;    //使用 (low+high) /2 可能会溢出。对于java，使用 mid =  (high+low)>>>1; 也可以防止溢出
            

            if (arr[mid] < target)
                low = mid + 1;
            else if (arr[mid] > target)
                high = mid - 1;
            
            else   //arr[mid] == target
                return mid; //  found
        }
        return -1; // not found.

}