Update 0090.子集II.md

修正python代码补充注释

Author: casnz1601

problems/0090.子集II.md

@@ -207,20 +207,30 @@ class Solution {
 Python：
 ```python3
 class Solution:
+    def __init__(self):
+        self.paths = []
+        self.path = []
+
     def subsetsWithDup(self, nums: List[int]) -> List[List[int]]:
-        res = []  #存放符合条件结果的集合
-        path = []  #用来存放符合条件结果
-        def backtrack(nums,startIndex):
-            res.append(path[:])
-            for i in range(startIndex,len(nums)):
-                if i > startIndex and nums[i] == nums[i - 1]:  #我们要对同一树层使用过的元素进行跳过
-                    continue
-                path.append(nums[i])
-                backtrack(nums,i+1)  #递归
-                path.pop()  #回溯
-        nums = sorted(nums)  #去重需要排序
-        backtrack(nums,0)
-        return res
+        nums.sort()
+        self.backtracking(nums, 0)
+        return self.paths
+
+    def backtracking(self, nums: List[int], start_index: int) -> None:
+        # ps.空集合仍符合要求
+        self.paths.append(self.path[:])
+        # Base Case
+        if start_index == len(nums):
+            return
+        
+        # 单层递归逻辑
+        for i in range(start_index, len(nums)):
+            if i > start_index and nums[i] == nums[i-1]:
+                # 当前后元素值相同时，跳入下一个循环，去重
+                continue
+            self.path.append(nums[i])
+            self.backtracking(nums, i+1)
+            self.path.pop()
 ```
 
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