LeetCode BinarySearch 702 Search in a Sorted Array of Unknown Size
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;
}
}
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