题目：

题解：

class Solution:
    def sumOfLeftLeaves(self, root: TreeNode) -> int:
        if not root:
            return 0
        
        isLeafNode = lambda node: not node.left and not node.right
        q = collections.deque([root])
        ans = 0

        while q:
            node = q.popleft()
            if node.left:
                if isLeafNode(node.left):
                    ans += node.left.val
                else:
                    q.append(node.left)
            if node.right:
                if not isLeafNode(node.right):
                    q.append(node.right)
        
        return ans