【leetcode】1213.Intersection of Three Sorted Arrays

题目如下：

Given three integer arrays arr1, arr2 and arr3 sorted in strictly increasing order, return a sorted array of only the integers that appeared in all three arrays.

Example 1:

Input: arr1 = [1,2,3,4,5], arr2 = [1,2,5,7,9], arr3 = [1,3,4,5,8]
Output: [1,5]
Explanation: Only 1 and 5 appeared in the three arrays.

Constraints:

• 1 <= arr1.length, arr2.length, arr3.length <= 1000
• 1 <= arr1[i], arr2[i], arr3[i] <= 2000

解题思路：和【leetcode&CN&竞赛】1198.Find Smallest Common Element in All Rows 类似，但是本题约定了每个数组中的元素是唯一的，所以只需要遍历三个数组，计算出每个元素出现的次数即可。

代码如下：

class Solution(object):
    def arraysIntersection(self, arr1, arr2, arr3):
        """
        :type arr1: List[int]
        :type arr2: List[int]
        :type arr3: List[int]
        :rtype: List[int]
        """
        val = [0] * 2001
        for i in arr1:
            val[i] += 1
        for i in arr2:
            val[i] += 1
        for i in arr3:
            val[i] += 1
        res = []
        for i,v in enumerate(val):
            if v == 3:res.append(i)
        return res