leetcode -- Sum Root to Leaf Numbers -- dfs

https://leetcode.com/problems/sum-root-to-leaf-numbers/

思路1

用dfs preorder求就行

class Solution(object):

    def dfs(self, root, subres, res):

        if root:
            if root.left == None and root.right == None:
                #print subres + str(root.val)
                res[0] += int(subres + str(root.val))
            else:
                self.dfs(root.left, subres + str(root.val), res)
                self.dfs(root.right, subres + str(root.val), res)

    def sumNumbers(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        res = [0]
        self.dfs(root, '', res)
        return res[0]

思路2

还提供了另一种递归的办法

class Solution:
    # @param root, a tree node
    # @return an integer
    def sum(self, root, preSum):#preSum记录root的父节点到global root的Sum。
        if root==None: return 0
        preSum = preSum*10 + root.val#这里跟位操作有点像，preSum向左移动一位，然后加上root.val。
        if root.left==None and root.right==None: return preSum
        return self.sum(root.left, preSum)+self.sum(root.right, preSum)

    def sumNumbers(self, root):
        return self.sum(root, 0)