[LeetCode&Python] Problem 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.

# 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 sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        # Recursion method
        '''
        def rer(node,left):
            if not node:
                return 0
            if left:
                if node.left or node.right:
                    return rer(node.left,True)+rer(node.right,False)
                else:
                    return node.val
            else:
                return rer(node.right,False)+rer(node.left,True)
        
        return rer(root,False)
        '''
        # Non-recursion method
        ans=0
        if root:
            stack=[root]
            
            while stack:
                node=stack.pop()
                if node.left:
                    if not node.left.left and not node.left.right:
                        ans+=node.left.val
                    else:
                        stack.append(node.left)
                if node.right:
                    stack.append(node.right)
        return ans