示例 1：
输入：arr = [2,3,1,6,7]输出：4解释：满足题意的三元组分别是 (0,1,2), (0,2,2), (2,3,4) 以及 (2,4,4)
来源：力扣（LeetCode）链接：https://leetcode-cn.com/problems/count-triplets-that-can-form-two-arrays-of-equal-xor著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

恒等律：X ⊕ 0 = X 
归零律：X ⊕ X = 0
交换律：A ⊕ B = B ⊕ A
结合律：A ⊕ (B ⊕ C) = (A ⊕ B) ⊕ C

    public int countTriplets(int[] arr) {        int n = arr.length;        int ans = 0;        for (int i = 0; i < n; i++) {            for (int j = i + 1; j < n; j++) {                for (int k = j; k < n; k++) {                    int a = 0, b = 0;                    for (int x = i; x < j; x++) {                        a ^= arr[x];                    }                    for (int y = j; y <= k; y++) {                        b ^= arr[y];                    }                    if (a == b) {                        ans++;                    }                }            }        }
        return ans;    }

    public int countTriplets(int[] arr) {        int n = arr.length;        int ans = 0;        int[] s = new int[n + 1];        for (int i = 0; i < n; i++) {            s[i + 1] = s[i] ^ arr[i];        }        for (int i = 0; i < n; i++) {            for (int j = i + 1; j < n; j++) {                for (int k = j; k < n; k++) {                    if (s[i] == s[k + 1]) {                        ans++;                    }                }            }        }
        return ans;    }