feat: add next permutation problem (#720)

README.md

@@ -834,6 +834,7 @@ Read our [Contribution Guidelines](CONTRIBUTING.md) before you contribute.
 
 1. [`GenerateElementSet`](./math/permutation/heaps.go#L37): No description provided.
 2. [`Heaps`](./math/permutation/heaps.go#L8):  Heap's Algorithm for generating all permutations of n objects
+3. [`NextPermutation`](./math/permutation/next_permutation.go#8): A solution to find next permutation of an integer array in constant memory
 
 ---
 </details><details>

math/permutation/next_permutation.go

@@ -0,0 +1,37 @@
+// A practice to find lexicographically next greater permutation of the given array of integers.
+// If there does not exist any greater permutation, then print the lexicographically smallest permutation of the given array.
+// The implementation below, finds the next permutation in linear time and constant memory and returns in place
+// Useful reference: https://www.geeksforgeeks.org/next-permutation/
+
+package permutation
+
+func NextPermutation(nums []int) {
+	pivot := 0
+	for pivot = len(nums) - 2; pivot >= 0; pivot-- {
+		if nums[pivot] < nums[pivot+1] {
+			break
+		}
+	}
+	if pivot < 0 {
+		// current permutation is the last and must be reversed totally
+		for l, r := 0, len(nums)-1; l < r; l, r = l+1, r-1 {
+			nums[l], nums[r] = nums[r], nums[l]
+		}
+	} else {
+		succ := 0
+		for succ = len(nums) - 1; succ > pivot; succ = succ - 1 {
+			if nums[succ] > nums[pivot] {
+				break
+			}
+		}
+
+		// Swap the pivot and successor
+		nums[pivot], nums[succ] = nums[succ], nums[pivot]
+
+		// Reverse the suffix part to minimize it
+		for l, r := pivot+1, len(nums)-1; l < r; l, r = l+1, r-1 {
+			nums[l], nums[r] = nums[r], nums[l]
+		}
+	}
+
+}

math/permutation/next_permutation_test.go

@@ -0,0 +1,40 @@
+package permutation
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestNextPermutation(t *testing.T) {
+	var nextPermutationTestData = []struct {
+		description string
+		numbers     []int
+		next        []int
+	}{
+		{
+			description: "Basic case",
+			numbers:     []int{1, 2, 3},
+			next:        []int{1, 3, 2},
+		},
+		{
+			description: "Should reverse the whole slice",
+			numbers:     []int{3, 2, 1},
+			next:        []int{1, 2, 3},
+		},
+		{
+			description: "A more complex test",
+			numbers:     []int{2, 4, 1, 7, 5, 0},
+			next:        []int{2, 4, 5, 0, 1, 7},
+		},
+	}
+	for _, test := range nextPermutationTestData {
+		t.Run(test.description, func(t *testing.T) {
+			NextPermutation(test.numbers)
+
+			if !reflect.DeepEqual(test.numbers, test.next) {
+				t.Logf("FAIL: %s", test.description)
+				t.Fatalf("Expected result:%v\nFound: %v", test.next, test.numbers)
+			}
+		})
+	}
+}