LintCode : Binary Tree Path Sum

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of binary tree
     * @param target an integer
     * @return all valid paths
     */
    private List<List<Integer>> result = new ArrayList<>();
    private ArrayList<Integer> path = new ArrayList<Integer>();
    public List<List<Integer>> binaryTreePathSum(TreeNode root, int target) {
        // Write your code here
        // handel corner cases
        if (root == null) {
            return result;
        }
        path.add(root.val);
        helper(root, path, target - root.val);
        return result;
    }
    
    private void helper(TreeNode node, ArrayList<Integer> path, int target) {
        if (node.left == null && node.right == null) {
            if (target == 0) {
                result.add(new ArrayList<Integer>(path));
            }
            return;
        }
        if (node.left != null) {
            path.add(node.left.val);
            helper(node.left, path, target - node.left.val);
            path.remove(path.size() - 1);
        }
        if (node.right != null) {
            path.add(node.right.val);
            helper(node.right, path, target - node.right.val);
            path.remove(path.size() - 1);
        }
    }
}