23.01.09 Today’s Leetcode

Binary Tree Preorder (easy)

# with DFS
class Solution:

    res = []
    
    def dfs(self, root):
        if root == None:
            return
        self.res.append(root.val)
        self.dfs(root.left)
        self.dfs(root.right)

    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        self.res = []
        self.dfs(root)
        return self.res

# with queue
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