404. Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

Solution:

class Solution(object):
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        right = self.sumOfLeftLeaves(root.right)
        if root.left and not root.left.left and not root.left.right:
            return root.left.val + right
        return self.sumOfLeftLeaves(root.left) + right