891 added

Author: aQua

Algorithms/0891.sum-of-subsequence-widths/README.md

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+# [891. Sum of Subsequence Widths](https://leetcode.com/problems/sum-of-subsequence-widths/)
+
+## 题目
+
+Given an array of integers A, consider all non-empty subsequences of A.
+
+For any sequence S, let thewidthof 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:
+
+```text
+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. 1 <= A.length <= 20000
+1. 1 <= A[i] <= 20000
+
+## 解题思路
+
+见程序注释

Algorithms/0891.sum-of-subsequence-widths/sum-of-subsequence-widths.go

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+package problem0891
+
+func sumSubseqWidths(A []int) int {
+
+	return 0
+}

Algorithms/0891.sum-of-subsequence-widths/sum-of-subsequence-widths_test.go

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+package problem0891
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// tcs is testcase slice
+var tcs = []struct {
+	A   []int
+	ans int
+}{
+
+	{
+		[]int{2, 1, 3},
+		6,
+	},
+
+	// 可以有多个 testcase
+}
+
+func Test_sumSubseqWidths(t *testing.T) {
+	ast := assert.New(t)
+
+	for _, tc := range tcs {
+		fmt.Printf("~~%v~~\n", tc)
+		ast.Equal(tc.ans, sumSubseqWidths(tc.A), "输入:%v", tc)
+	}
+}
+
+func Benchmark_sumSubseqWidths(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range tcs {
+			sumSubseqWidths(tc.A)
+		}
+	}
+}