Added statistical method to ints package.

ints/statistics.go

@@ -0,0 +1,47 @@
+package ints
+
+// Faculty returns the faculty of i.
+func Faculty(i int) int {
+	if i < 2 {
+		return 1
+	}
+	return i * Faculty(i-1)
+}
+
+// Faculty64 returns the faculty of i.
+func Faculty64(i int64) int64 {
+	if i < 2 {
+		return 1
+	}
+	return i * Faculty64(i-1)
+}
+
+// Permutations returns all possible permutations of the the set integers.
+func Permutations(set []int) [][]int {
+	var n = len(set)
+	if n == 0 {
+		return nil
+	}
+	var res = make([][]int, 0, Faculty(n))
+	var p = append([]int(nil), set...)
+
+	var c = make([]int, n)
+	res = append(res, append([]int(nil), p...))
+	i := 0
+	for i < n {
+		if c[i] < i {
+			if i%2 == 0 {
+				p[0], p[i] = p[i], p[0]
+			} else {
+				p[c[i]], p[i] = p[i], p[c[i]]
+			}
+			res = append(res, append([]int(nil), p...))
+			c[i] = c[i] + 1
+			i = 0
+		} else {
+			c[i] = 0
+			i++
+		}
+	}
+	return res
+}