feat(0031_next-permutation): add py3 soln

0031_next-permutation/python3/solution.py

@@ -0,0 +1,35 @@
+class Solution:
+    def nextPermutation(self, nums: List[int]) -> None:
+        """
+        Do not return anything, modify nums in-place instead.
+        """
+        # Find start of strictly decreasing section from the right
+        # and identify the position of element right before it
+        
+        right = len(nums) - 1
+        
+        while right > 0:
+            if nums[right - 1] < nums[right]:
+                break
+            
+            right -= 1
+        
+        swap_pos = right - 1
+        
+        # Array is already in decreasing order, hence we return the reverse
+        if swap_pos < 0:
+            nums.reverse()
+            return
+        
+        # Find next greatest element from [swap_pos + 1, n)
+        right = len(nums) - 1
+        while right > swap_pos:
+            if nums[right] > nums[swap_pos]:
+                break
+            
+            right -= 1
+        
+        nums[swap_pos], nums[right] = nums[right], nums[swap_pos]
+        
+        # Reverse [swap_pos + 1, n)
+        nums[swap_pos + 1:] = nums[:swap_pos:-1]