23.02.16 Today’s leetcode

144. Binary Tree Preorder Traversal (easy)

# 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 preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        
        ans, stack = [] , []
        
        stack.append(root)
        
        while stack:
            
            curNode = stack.pop()
            if curNode:
                ans.append(curNode.val)
                stack.append(curNode.right)
                stack.append(curNode.left)
        
        return ans

94. Binary Tree Inorder Traversal (easy)

# 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]:
        ans = []
        def dfs(node):
            if not node:
                return
            dfs(node.left)
            ans.append(node.val)
            dfs(node.right)
        dfs(root)
        return ans

145. Binary Tree Postorder Traversal (easy)

# 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 postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        ans = []
        def dfs(node):
            if not node:
                return
            dfs(node.left)
            dfs(node.right)
            ans.append(node.val)
        dfs(root)
        return ans