# 1497. Check If Array Pairs Are Divisible by k

Given an array of integers `arr` of even length `n` and an integer `k`.

We want to divide the array into exactly `n / 2` pairs such that the sum of each pair is divisible by `k`.

Return *True* If you can find a way to do that or *False* otherwise.

**Example 1:**

```
Input: arr = [1,2,3,4,5,10,6,7,8,9], k = 5
Output: true
Explanation: Pairs are (1,9),(2,8),(3,7),(4,6) and (5,10).
```

**Example 2:**

```
Input: arr = [1,2,3,4,5,6], k = 7
Output: true
Explanation: Pairs are (1,6),(2,5) and(3,4).
```

**Example 3:**

```
Input: arr = [1,2,3,4,5,6], k = 10
Output: false
Explanation: You can try all possible pairs to see that there is no way to divide arr into 3 pairs each with sum divisible by 10.
```

**Example 4:**

```
Input: arr = [-10,10], k = 2
Output: true
```

**Example 5:**

```
Input: arr = [-1,1,-2,2,-3,3,-4,4], k = 3
Output: true
```

**Constraints:**

* `arr.length == n`
* `1 <= n <= 10^5`
* `n` is even.
* `-10^9 <= arr[i] <= 10^9`
* `1 <= k <= 10^5`

问给定偶数个数目的数组能否俩俩配对，每对和都是K的倍数。和是K的倍数 (a + b) % K == 0 =>取余后和mod K为0 (a % K + b % K) % K == 0 。统计余数a出现的频率，看K - a的频率是否与之相等。余数为0时满足频率为偶数。

```python
class Solution:
    def canArrange(self, arr: List[int], k: int) -> bool:
        m = collections.defaultdict(int)
        for a in arr:
            m[a % k] += 1
        for key, value in m.items():
            if key == 0:
                if m[0] % 2 != 0: return False
                continue
            if m[k - key] != value: return False
        return True
```


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