702. Search in a Sorted Array of Unknown Size - Medium

Given an integer array sorted in ascending order, write a function to search target in nums.  If target exists, then return its index, otherwise return -1. However, the array size is unknown to you. You may only access the array using an ArrayReader interface, where ArrayReader.get(k) returns the element of the array at index k (0-indexed).

You may assume all integers in the array are less than 10000, and if you access the array out of bounds, ArrayReader.get will return 2147483647.

 

Example 1:

Input: array = [-1,0,3,5,9,12], target = 9
Output: 4
Explanation: 9 exists in nums and its index is 4

Example 2:

Input: array = [-1,0,3,5,9,12], target = 2
Output: -1
Explanation: 2 does not exist in nums so return -1

 

Note:

1. You may assume that all elements in the array are unique.
2. The value of each element in the array will be in the range [-9999, 9999].

 

先向外binary search找到右边界right，再向内binary search

time: O(log_2(n)), space: O(1)

class Solution {
    public int search(ArrayReader reader, int target) {
        int left = 0, right = 1;
        while(reader.get(right) != 2147483647) {
            right *= 2;
        }
        
        while(left <= right) {
            int mid = left + (right - left) / 2;
            if(reader.get(mid) == target)
                return mid;
            else if(reader.get(mid) > target)
                right = mid - 1;
            else
                left = mid + 1;
        }
        return -1;
    }
}