feat(solutions): add permutation_sequence

Author: WindomZ

solutions/permutation_sequence/README.md

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+# 60. Permutation Sequence
+
+## Description
+
+The set `[1,2,3,…,_n_]` contains a total of _n_! unique permutations.
+
+By listing and labeling all of the permutations in order,  
+
+We get the following sequence (ie, for _n_ = 3):
+
+1.  `“123“`
+2.  `“132“`
+3.  `“213“`
+4.  `“231“`
+5.  `“312“`
+6.  `“321“`
+
+Given _n_ and _k_, return the _k_th permutation sequence.
+
+**Note:** Given _n_ will be between 1 and 9 inclusive.

solutions/permutation_sequence/permutationsequence.go

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+package permutationsequence
+
+func getPermutation(n int, k int) string {
+	if k <= 0 {
+		return ""
+	}
+	s := make([]byte, n)
+	i, j, f := 1, 0, 1
+	for ; i <= n; i++ {
+		f *= i
+		s[i-1] += byte('0' + i) // make s become 1234...n
+	}
+	for i, k = 0, k-1; i < n; i++ {
+		f /= n - i
+		j = i + k/f // calculate and put at s[i]
+		temp := s[j]
+		// remove c by shifting to cover up (adjust the right part).
+		for ; j > i; j-- {
+			s[j] = s[j-1]
+		}
+		k %= f
+		s[i] = temp
+	}
+	return string(s)
+}

solutions/permutation_sequence/permutationsequence_test.go

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+package permutationsequence
+
+import (
+	"testing"
+
+	"github.com/WindomZ/testify/assert"
+)
+
+func TestGetPermutation(t *testing.T) {
+	assert.Equal(t, "", getPermutation(0, 0))
+	assert.Equal(t, "1", getPermutation(1, 1))
+	assert.Equal(t, "123", getPermutation(3, 1))
+	assert.Equal(t, "132", getPermutation(3, 2))
+	assert.Equal(t, "213", getPermutation(3, 3))
+	assert.Equal(t, "231", getPermutation(3, 4))
+	assert.Equal(t, "312", getPermutation(3, 5))
+	assert.Equal(t, "321", getPermutation(3, 6))
+}
+
+func Benchmark_getPermutation(b *testing.B) {
+	b.StopTimer()
+	b.ReportAllocs()
+	b.StartTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			getPermutation(0, 0)
+			getPermutation(1, 1)
+			getPermutation(3, 1)
+			getPermutation(3, 2)
+			getPermutation(3, 3)
+			getPermutation(3, 6)
+		}
+	})
+}