Create 32. 二叉树的中序遍历.md

Author: jasoncao11

32. 二叉树的中序遍历.md

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+***给定一个二叉树的根节点 root ，返回它的中序遍历 。***
+
+```
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
+        #思路：栈，中序遍历：左，中，右，那么入栈顺序必须调整为倒序，也就是右，中，左
+        UNUSED, USED = 0, 1
+        stack=[(UNUSED, root)]
+        res = []
+        while stack:
+            state, node = stack.pop()
+            if not node:
+                continue
+            if state == 0:
+                stack.append((UNUSED, node.right))
+                stack.append((USED, node))
+                stack.append((UNUSED, node.left))
+            else:
+                res.append(node.val)
+        return res  
+```