Update 031-Next Permutation

Changes:
----------
* Devided resposiblity with using functions.
* Slice in already a reference type in go, means in passes actual slice(not a copy) to functions, that's why using pointers in swap and reverse functions look redundant in my opinion.

Discussion:
--------------
https://leetcode.com/problems/next-permutation/discuss/1554932/Go-Submission-with-Explanation

Author: Ashef Habib Tishad

leetcode/0031.Next-Permutation/31. Next Permutation.go

@@ -1,31 +1,55 @@
+// https://leetcode.com/problems/next-permutation/discuss/1554932/Go-Submission-with-Explanation
+// Time O(N) , Space: O(1)
+
 package leetcode
 
+// [2,(3),6,5,4,1] -> 2,(4),6,5,(3),1 -> 2,4, 1,3,5,6
 func nextPermutation(nums []int) {
-	i, j := 0, 0
-	for i = len(nums) - 2; i >= 0; i-- {
-		if nums[i] < nums[i+1] {
-			break
-		}
+	var n = len(nums)
+	var pIdx = checkPermutationPossibility(nums)
+	if pIdx == -1 {
+		reverse(nums, 0, n-1)
+		return
 	}
-	if i >= 0 {
-		for j = len(nums) - 1; j > i; j-- {
-			if nums[j] > nums[i] {
-				break
-			}
+
+	var rp = len(nums) - 1
+	// start from right most to leftward,find the first number which is larger than PIVOT
+	for rp > 0 {
+		if nums[rp] > nums[pIdx] {
+			swap(nums, pIdx, rp)
+			break
+		} else {
+			rp--
 		}
-		swap(&nums, i, j)
 	}
-	reverse(&nums, i+1, len(nums)-1)
+	// Finally, Reverse all elements which are right from pivot
+	reverse(nums, pIdx+1, n-1)
 }
 
-func reverse(nums *[]int, i, j int) {
-	for i < j {
-		swap(nums, i, j)
-		i++
-		j--
+func swap(nums []int, i, j int) {
+	nums[i], nums[j] = nums[j], nums[i]
+}
+
+func reverse(nums []int, s int, e int) {
+	for s < e {
+		swap(nums, s, e)
+		s++
+		e--
 	}
 }
 
-func swap(nums *[]int, i, j int) {
-	(*nums)[i], (*nums)[j] = (*nums)[j], (*nums)[i]
+// checkPermutationPossibility returns 1st occurrence Index where
+// value is in decreasing order(from right to left)
+// returns -1 if not found(it's already in its last permutation)
+func checkPermutationPossibility(nums []int) (idx int) {
+	// search right to left for 1st number(from right) that is not in increasing order
+	var rp = len(nums) - 1
+	for rp > 0 {
+		if nums[rp-1] < nums[rp] {
+			idx = rp - 1
+			return idx
+		}
+		rp--
+	}
+	return -1
 }