3681. Maximum XOR of Subsequences
Description
You are given an integer array nums of length n where each element is a non-negative integer.
Select two subsequences of nums (they may be empty and are allowed to overlap), each preserving the original order of elements, and let:
Xbe the bitwise XOR of all elements in the first subsequence.Ybe the bitwise XOR of all elements in the second subsequence.
Return the maximum possible value of X XOR Y.
Note: The XOR of an empty subsequence is 0.
Example 1:
Input: nums = [1,2,3]
Output: 3
Explanation:
Choose subsequences:
- First subsequence
[2], whose XOR is 2. - Second subsequence
[2,3], whose XOR is 1.
Then, XOR of both subsequences = 2 XOR 1 = 3.
This is the maximum XOR value achievable from any two subsequences.
Example 2:
Input: nums = [5,2]
Output: 7
Explanation:
Choose subsequences:
- First subsequence
[5], whose XOR is 5. - Second subsequence
[2], whose XOR is 2.
Then, XOR of both subsequences = 5 XOR 2 = 7.
This is the maximum XOR value achievable from any two subsequences.
Constraints:
2 <= nums.length <= 1050 <= nums[i] <= 109
Solutions
Solution 1
Thinking
A subsequence XOR is a subset XOR. The maximum is given by a linear basis: insert every value, then greedy from the high bit. \(n\le 10^5\) is fine.
From the high bit downward, XOR the basis vector in when it enlarges the answer. The empty subset is \(0\) and a singleton is available, so the result is nonnegative.
Each bit keeps at most one basis vector; insert and query are \(O(\log A)\).
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