Typical recursive DFS.

Space: O(n) due to recursion.

Time: O(n^2) in worst case (no branching); O(nlogn) in best case (balanced tree).

```
public class Solution {
public int pathSum(TreeNode root, int sum) {
if (root == null) return 0;
return pathSumFrom(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum);
}
private int pathSumFrom(TreeNode node, int sum) {
if (node == null) return 0;
return (node.val == sum ? 1 : 0)
+ pathSumFrom(node.left, sum - node.val) + pathSumFrom(node.right, sum - node.val);
}
}
```