1442. Count Triplets That Can Form Two Arrays of Equal XOR

Given an array of integers arr.

We want to select three indices i, j and k where (0 <= i < j <= k < arr.length).

Let's define a and b as follows:

• a = arr[i] ^ arr[i + 1] ^ ... ^ arr[j - 1]
• b = arr[j] ^ arr[j + 1] ^ ... ^ arr[k]

Note that ^ denotes the bitwise-xor operation.

Return the number of triplets (i, j and k) Where a == b.

 

Example 1:

Input: arr = [2,3,1,6,7]
Output: 4
Explanation: The triplets are (0,1,2), (0,2,2), (2,3,4) and (2,4,4)

Example 2:

Input: arr = [1,1,1,1,1]
Output: 10

Example 3:

Input: arr = [2,3]
Output: 0

Example 4:

Input: arr = [1,3,5,7,9]
Output: 3

Example 5:

Input: arr = [7,11,12,9,5,2,7,17,22]
Output: 8

 

Constraints:

• 1 <= arr.length <= 300
• 1 <= arr[i] <= 10^8

思路：找出异或和为0的区间

class Solution(object):
    def countTriplets(self, arr):
        """
        :type arr: List[int]
        :rtype: int
        """
        from collections import defaultdict
        d = defaultdict(list)
        d[0].append(-1)
        res = 0
        s = 0
        for j in range(len(arr)):
            s ^= arr[j]
            for i in d[s]:
                res += j-i-1
            d[s].append(j)
        return res