二分法模板

        int left = 0;
        int right = nums.length - 1;
        int res = -1;
        while(left <= right){
	     //注意这里可以使用 right + (left - right)/2来防止数字溢出
            //相同的 right + (left - right) >> 1
            int mid = (right + left)/2;
            if(nums[mid] == target)res = mid;
            else if(nums[mid] > target){
                right = mid - 1;
            }else{
                left = mid + 1;
            }
        }
        return res;