update python 105

python-leetcode/solution/tree/leetcode_105_.py

@@ -20,3 +20,17 @@ class Solution:
         root.left = self.buildTree(preorder[1:k + 1], inorder[:k])
         root.right = self.buildTree(preorder[k + 1:], inorder[k + 1:])
         return root
+
+    ##优化版
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
+        def helper(preorder: List[int], inorder: List[int], preleft: int, preright: int, inleft: int, inright: int):
+            if inleft > inright:
+                return None
+            val = preorder[preleft]
+            mid = inorder.index(val)
+            root = TreeNode(val)
+            root.left = helper(preorder, inorder, preleft + 1, mid - inleft + preleft, inleft, mid - 1)
+            root.right = helper(preorder, inorder, mid - inleft + preleft + 1, preright, mid + 1, inright)
+            return root
+
+        return helper(preorder, inorder, 0, len(preorder) - 1, 0, len(inorder) - 1)