257_Binary_Tree_Paths.py

# 2 Possible Solutions
# 1. Recursion
# 2. Iteration

# 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:
    # Recursion
    # Time: O(N), Space: O(log N)/N
    def binaryTreePaths(self, root: Optional[TreeNode]) -> List[str]:
        def construct_paths(root, path):
            if root:
                path += str(root.val)
                if not root.left and not root.right:  # if reach a leaf
                    paths.append(path)  # update paths  
                else:
                    path += '->'  # extend the current path
                    construct_paths(root.left, path)
                    construct_paths(root.right, path)

        paths = []
        construct_paths(root, '')
        return paths

    # Iteration
    # Time: O(N), Space: O(log N)/N
    def binaryTreePaths(self, root: Optional[TreeNode]) -> List[str]:
        if not root: 
            return []
        
        paths = []
        stack = [(root, str(root.val))]
        
        while stack:
            node, path = stack.pop()
            if not node.left and not node.right:
                paths.append(path)
            if node.left:
                stack.append((node.left, path + "->" + str(node.left.val)))
            if node.right:
                stack.append((node.right, path + "->" + str(node.right.val)))
            
        return paths