DFS
class Solution:
# @param {TreeNode} root
# @param {integer} sum
# @return {integer[][]}
def pathSum(self, root, sum):
    ans = []
    self.dfs(root, sum, [],ans)
    return ans

def dfs(self, root, sum, tmp, ans):
    if not root:
        return

    if root.left == None and root.right == None and sum == root.val:
        ans.append(tmp+[root.val])
        return

    self.dfs(root.left, sum-root.val, tmp+[root.val], ans)
    self.dfs(root.right, sum-root.val, tmp+[root.val], ans)

time and space complex: O(2^n)