LC404. Sum of Left Leaves

# 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 sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        def dfs(node, is_left = False, total = 0):
            if not node:
                return total
            
            if not node.left and not node.right:
                if is_left:
                    return total + node.val
                else:
                    return total
            
            return total + dfs(node.left, True) + dfs(node.right)
        
        return dfs(root) 

Complexity

Time: $O(n)$
Space: $O(n)$