permutation

Author: wuduhren

problems/python3/permutations.py

@@ -0,0 +1,28 @@
+"""
+Time: O(N!), since we call helper() N! times.
+Space: O(N) for recursion stacks.
+
+Whenever we call `helper()`, we pick a num that is not "used" and add it to the `permutation`.
+Recursively call the `helper()` until we filled the `permutation`.
+Resotre `permutation` and `used` and try another `num`.
+"""
+class Solution:
+    def permute(self, nums: List[int]) -> List[List[int]]:
+        def helper():
+            if len(permutation)==len(nums):
+                ans.append(permutation.copy())
+                return
+            
+            for num in nums:
+                if num in used: continue
+                used.add(num)
+                permutation.append(num)
+                helper()
+                used.remove(num)
+                permutation.pop()
+        
+        ans = []
+        permutation = []
+        used = set()
+        helper()
+        return ans