[Leetcode/Tree] 404. 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:
        if not root:
            return 0
        
        total = 0

        # 왼쪽 자식이 리프 노드라면 값 추가
        if root.left and not root.left.left and not root.left.right:
            total += root.left.val
        
        # 왼쪽과 오른쪽 서브트리 탐색 (재귀 호출)
        total += self.sumOfLeftLeaves(root.left)
        total += self.sumOfLeftLeaves(root.right)

        return total