Solution 4: 二叉树的路径和

public class TreePathSum {
	public List<List<Integer>> getTreePathSum(TreeNode root, int sum) {
		List<List<Integer>> res = new ArrayList<List<Integer>>();
		if (root == null) {
			return res;
		}
		List<Integer> path = new ArrayList<Integer>();
		helper(root, sum, path, res);

		return res;
	}

	private void helper(TreeNode root, int sum, List<Integer> path,
			List<List<Integer>> res) {
		if (root == null) {
			return;
		}
		if (sum == root.val) {
			path.add(root.val);
			res.add(new ArrayList<Integer>(path));
			path.remove(path.size()-1);
			return;
		}
		
		path.add(root.val);
		helper(root.left, sum - root.val, path, res);
		helper(root.right, sum - root.val, path, res);
		path.remove(path.size() - 1);
	}
}