LeetCode: Sum of Left Leaves
Find the sum of all left leaves in a given binary tree.

  3
   / \
  9  20
    /  \
   15   7

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

Return 24.

#Two key points to note here: Left branch and Leaf nodes

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def __init__(self):
        self.sumLeft = 0 # declare a global variable to account for the sum of left branches
        
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.findLeftSum(root)
        #print 'sum: ', self.sumLeft
        return self.sumLeft

        
    def findLeftSum(self, node, dir=None):
        #////////////////////////////////
        # do it using in-order traversal
        #////////////////////////////////
        if node is None:
            return 0

        # Only sum those node vals when the dir = 0 (signifying left branch) and
        # it has no more child nodes
        if dir == 0 and node.left is None and node.right is None:
            self.sumLeft += node.val
            
        # When left node is pushed to the stack, pass it's identifier direction as 0
        self.findLeftSum(node.left, 0)
        #print 'node val: ', node.val
        self.findLeftSum(node.right, 1)