二分查找

private int binarySearch(int[] nums,int key) {
        int left = 0;
        int right = nums.length - 1;
        int mid;
        while(left <= right){
            mid = (left + right)/2;
            if(nums[mid] < key){
                left = mid + 1;
            }else if(nums[mid] > key){
                right = mid - 1;
            }else{
                return mid;
            }
        }
        return -1;
    }

时间复杂度：O(logn)