LeetCode437题：路径总和III（python3）

437:路径总和III

整条路径和减去target和，如果存在这样的一个前缀节点，那count+1，遍历满足等式的，就可以加count

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def pathSum(self, root: Optional[TreeNode], targetSum: int) -> int:
        def dfs(node, cnts, cursum):
            if not node:
                return 0
            cursum += node.val #前缀和
            ans = cnts[cursum - targetSum] #前缀和-target
            cnts[cursum] += 1 #有就加count
            if node.left:
                ans += dfs(node.left, cnts, cursum)
            if node.right:
                ans += dfs(node.right, cnts, cursum)
            cnts[cursum] -= 1
            return ans
        return dfs(root, Counter([0]), 0)
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