You are given an integer array nums of length n, and a positive integer k.
The power of a subsequence is defined as the minimum absolute difference between any two elements in the subsequence.
\n\nReturn the sum of powers of all subsequences of nums which have length equal to k.
Since the answer may be large, return it modulo 109 + 7.
\n
Example 1:
\n\nInput: nums = [1,2,3,4], k = 3
\n\nOutput: 4
\n\nExplanation:
\n\nThere are 4 subsequences in nums which have length 3: [1,2,3], [1,3,4], [1,2,4], and [2,3,4]. The sum of powers is |2 - 3| + |3 - 4| + |2 - 1| + |3 - 4| = 4.
Example 2:
\n\nInput: nums = [2,2], k = 2
\n\nOutput: 0
\n\nExplanation:
\n\nThe only subsequence in nums which has length 2 is [2,2]. The sum of powers is |2 - 2| = 0.
Example 3:
\n\nInput: nums = [4,3,-1], k = 2
\n\nOutput: 10
\n\nExplanation:
\n\nThere are 3 subsequences in nums which have length 2: [4,3], [4,-1], and [3,-1]. The sum of powers is |4 - 3| + |4 - (-1)| + |3 - (-1)| = 10.
\n
Constraints:
\n\n2 <= n == nums.length <= 50-108 <= nums[i] <= 108 2 <= k <= n给你一个长度为 n 的整数数组 nums 和一个 正 整数 k 。
一个子序列的 能量 定义为子序列中 任意 两个元素的差值绝对值的 最小值 。
\n\n请你返回 nums 中长度 等于 k 的 所有 子序列的 能量和 。
由于答案可能会很大，将答案对 109 + 7 取余 后返回。
\n\n
示例 1：
\n\n输入：nums = [1,2,3,4], k = 3
\n\n输出：4
\n\n解释：
\n\nnums 中总共有 4 个长度为 3 的子序列：[1,2,3] ，[1,3,4] ，[1,2,4] 和 [2,3,4] 。能量和为 |2 - 3| + |3 - 4| + |2 - 1| + |3 - 4| = 4 。
示例 2：
\n\n输入：nums = [2,2], k = 2
\n\n输出：0
\n\n解释：
\n\nnums 中唯一一个长度为 2 的子序列是 [2,2] 。能量和为 |2 - 2| = 0 。
示例 3：
\n\n输入：nums = [4,3,-1], k = 2
\n\n输出：10
\n\n解释：
\n\nnums 总共有 3 个长度为 2 的子序列：[4,3] ，[4,-1] 和 [3,-1] 。能量和为 |4 - 3| + |4 - (-1)| + |3 - (-1)| = 10 。
\n\n
提示：
\n\n2 <= n == nums.length <= 50-108 <= nums[i] <= 108 2 <= k <= nnums.",
"There are at most n2 distinct differences.",
"For a particular difference d, let dp[len][i][j] be the number of subsequences of length len in the subarray nums[0..i] where the last element picked was at index j.",
"For each index, we can check if it can be picked if nums[i] - nums[j] <= d."
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