LeetCode 437. Path Sum III

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题目

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.

Example:

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root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8

      10
     /  \
    5   -3
   / \    \
  3   2   11
 / \   \
3  -2   1

Return 3. The paths that sum to 8 are:

1.  5 -> 3
2.  5 -> 2 -> 1
3. -3 -> 11

思路

DFS.

代码

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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: int
        """
        if not root: return 0
        return self.dfs(root, sum) + self.pathSum(root.left, sum) + self.pathSum(root.right, sum)
    
    def dfs(self, root, sum):
        res = 0
        if not root: return res
        sum -= root.val
        if sum == 0:
            res += 1
        res += self.dfs(root.left, sum)
        res += self.dfs(root.right, sum)
        return res