404. Sum of Left Leaves

# Easy

Two methods:

1. Use stack<node, is_left>, while loop, very straight forward

2. Use recursive, but is harder

Python(I)

class Solution:
    def sumOfLeftLeaves(self, root: TreeNode) -> int:
        # edge case
        if root == None:
            return 0
        
        # regular case
        if root.left == None:
            return self.sumOfLeftLeaves(root.right)

        if root.left.left == None and root.left.right == None:
            return root.left.val + self.sumOfLeftLeaves(root.right)
        
        return self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right)

Python(II)

class Solution:
    def sumOfLeftLeaves(self, root: TreeNode) -> int:
        # edge case
        if root == None:
            return 0
        
        # regular case
        stack = [(root, False)]
        result = 0
        while len(stack) != 0:
            root, is_left = stack.pop()
            if root.left == None and root.right == None and is_left:
                result = result + root.val
            if root.left != None:
                stack.append((root.left, True))
            if root.right != None:
                stack.append((root.right, False))
        return result