题目：

题解：

func pathSum(root *TreeNode, targetSum int) (ans int) {
    preSum := map[int64]int{0: 1}
    var dfs func(*TreeNode, int64)
    dfs = func(node *TreeNode, curr int64) {
        if node == nil {
            return
        }
        curr += int64(node.Val)
        ans += preSum[curr-int64(targetSum)]
        preSum[curr]++
        dfs(node.Left, curr)
        dfs(node.Right, curr)
        preSum[curr]--
        return
    }
    dfs(root, 0)
    return
}