Sum of Left Leaves

class Solution:
    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        def traverse(node, pre):
            if not node:
                return 0
            csum = 0
            if (not node.left) and (not node.right) and pre == 'l':
                csum = node.val
            if node.left:
                csum = csum + traverse(node.left, 'l')
            if node.right:
                csum = csum + traverse(node.right, 'r')
            return csum
        
        return traverse(root, 'c')