一个小问题113. Path Sum II

请问为什么找到了一条符合条件的root to leaf path之后不return呢？
也就是if (pathSum == target){ res.add(new ArrayList<Integer>(list)); }
这里为什么不直接return了？我一开始写写了return 然后就错了

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null){
            return res;
        }
        List<Integer> list = new ArrayList<>();
        helper(res, list, root, sum);
        return res;
    }
    
    private void helper(List<List<Integer>> res, List<Integer> list, TreeNode root, int target){
        if (root == null){
            return;
        }
        list.add(root.val);
        if (root.left == null && root.right == null){
            int pathSum = 0;
            for (int i = 0; i < list.size(); i++){
                pathSum += list.get(i);
            }
            if (pathSum == target){
                res.add(new ArrayList<Integer>(list));
            }
        }
        helper(res, list, root.left, target);
        helper(res, list, root.right, target);
        list.remove(list.size() - 1);
    }
}