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LeetCode BinarySearch 702 Search in a Sorted Array of Unknown Size

程序员文章站 2024-01-13 22:25:46
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702. Search in a Sorted Array of Unknown Size

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:

首先采用二分法的前提条件是确定边界,然而这题由于是无限大的array,无法直接确定,边界,所以首次采用倍增法,大致确定左右边界,然后再用二分法找到target位置。

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