Leetcode 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.






 1 class Solution(object):
 2     def sumOfLeftLeaves(self, root):
 3         """
 4         :type root: TreeNode
 5         :rtype: int
 6         """
 7         ans = []
 8         if not root:
 9             return 0
10         self.sumLeft(root, ans)
11         return sum(ans)
12         
13     def sumLeft(self, node, ans):
14         if node.left is not None: 
15             if node.left.right is None and node.left.left is None:
16                 ans.append(node.left.val)
17             else:
18                 self.sumLeft(node.left, ans)
19         
20         if node.right != None:
21             self.sumLeft(node.right, ans)




注意： 如果想在辅助函数里维护一个变量为sum of left leaves。这个变量需要是一个全局变量。


 


可以用如下似乎递归求解。




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




 






            

            
posted @ 
2016-12-27 10:13 
lettuan 
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