二叉树遍历

二叉树的遍历

1. 前序遍历

1.1 递归前序遍历

   def preorderTraversal(self, root: TreeNode) -> List[int]:
       if not root:
           return []
       return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)

1.2 非递归前序遍历

    def preorderTraversal(self, root: TreeNode) -> List[int]:
        dummy = TreeNode(0)
        dummy.right = root
        res, stack = [], [dummy]
        while stack:
            node = stack.pop().right
            while node:
                res.append(node.val)
                stack.append(node)
                node = node.left
        return res

2 中序遍历

2.1递归遍历

    def inorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)

2.2非递归中序遍历

    def inorderTraversal(self, root: TreeNode) -> List[int]:
        dummy = TreeNode(0)
        dummy.right = root
        res, stack = [], [dummy]
        while stack:
            node = stack.pop()
            res.append(node.val)
            node = node.right
            while node:
                stack.append(node)
                node = node.left
        return res[1:]

3 后序遍历

3.1 递归后序遍历

    def postorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        return self.postorderTraversal(root.left) + self.postorderTraversal(root.right) +[root.val]

3.2 非递归后序遍历

    def postorderTraversal(self, root: TreeNode) -> List[int]:
        dummy = TreeNode(0)
        dummy.right = root
        
        res, stack, pre = [], [dummy], None
        while stack:
            node = stack[-1]
            if node.right and pre != node.right:
                node = node.right
                while node:
                    stack.append(node)
                    node = node.left
            else:
                res.append(stack.pop().val)
                pre = node
        return res[:-1]