LeetCode #702 - Search in a Sorted Array of Unknown Size

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本文介绍了一种在未知大小的有序数组中使用二分搜索查找目标元素的方法。通过不断扩展搜索范围直到找到目标元素或确定元素不存在，该算法在O(log n)时间内高效解决问题。文章提供了详细的实现步骤和示例。

题目描述：

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].

// Forward declaration of ArrayReader class.
class ArrayReader;

class Solution {
public:
    int search(const ArrayReader& reader, int target) {
        int begin=0;
        int end=1;
        while(reader.get(end)<target) end*=2;
        while(begin<=end)
        {
            int mid=(begin+end)/2;
            if(reader.get(mid)==target) return mid;
            else if(reader.get(mid)>target) end=mid-1;
            else begin=mid+1;
        }
        return -1;
    }
};