[LeetCode] 702. Search in a Sorted Array of Unknown Size

Problem

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:

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

Solution

class Solution {
    public int search(ArrayReader reader, int target) {
        //find higher bound
        int r = 1;
        while (reader.get(r) < target) r *= 2;
        //then you know lower bound
        int l = r/2;
        while (l <= r) {
            int m = l+(r-l)/2;
            if (reader.get(m) == target) return m;
            else if (reader.get(m) < target) l = m+1;
            else r = m-1;
        }
        return -1;
    }
}