路径总和 III

def solve(____):
    from collections import defaultdict
    count = ____
    prefix = defaultdict(____)
    prefix[____] = ____
    def dfs(____):
        nonlocal count
        if not node:
            return
        curr += node.val
        count += prefix[curr - targetSum]
        prefix[curr] += ____
        dfs(____)
        dfs(____)
        prefix[curr] -= ____
    dfs(____)
    return count