891. Sum of Subsequence Widths

Given an array of integers A, consider all non-empty subsequences of A.

For any sequence S, let the width of S be the difference between the maximum and minimum element of S.

Return the sum of the widths of all subsequences of A.

As the answer may be very large, return the answer modulo 10^9 + 7.

Example 1:

Input: [2,1,3]
Output: 6
Explanation: Subsequences are [1], [2], [3], [2,1], [2,3], [1,3], [2,1,3].
The corresponding widths are 0, 0, 0, 1, 1, 2, 2.
The sum of these widths is 6.

Note:

• 1 <= A.length <= 20000
• 1 <= A[i] <= 20000

# @lc code=start
using LeetCode

function sum_subseq_widths(A::Vector{Int})
    MOD = Int(1e9 + 7)
    len = length(A)
    sort!(A)
    pow = fill(2, len)
    for i in 2:len
        pow[i] = (pow[i - 1] << 1) % MOD
    end
    res = 0
    for i in 2:(len - 1)
        res += (A[i]) * ((pow[i - 1] - pow[len - i])) % MOD
    end
    return mod1((res + (A[end] - A[1]) * (pow[len - 1] - 1) % MOD), MOD)
end
# @lc code=end

sum_subseq_widths (generic function with 1 method)

This page was generated using DemoCards.jl and Literate.jl.