Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example: Given the below binary tree andsum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        helper(root, new ArrayList<Integer>(), ans, 0, sum);
        return ans;
    }
    private void helper(TreeNode root, List<Integer> path, List<List<Integer>> ans, int sum, int target) {
        path.add(root.val);
        sum += root.val;
        if (root.left == null && root.right == null) {
            if (sum == target) {
                ans.add(new ArrayList<Integer>(path));
            }
           // path.remove(path.size() - 1);
          //  return;     这两句可加可不加
        }

        if (root.left != null) {
            helper(root.left, path, ans, sum, target);
        }
        if (root.right != null) {
            helper(root.right, path, ans, sum, target);
        }
        path.remove(path.size() - 1);
    }

或者这样写：

    public List<List<Integer>> binaryTreePathSum(TreeNode root, int target) {
        // Write your code here
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null){
            return res;
        }
        List<Integer> path = new ArrayList<Integer>();
        path.add(root.val);
        helper(root, root.val, target, path, res);
        return res;
    }

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

        if(root.left != null){
            path.add(root.left.val);
            helper(root.left, sum + root.left.val, target, path, res);
            path.remove(path.size() - 1);
        }
        if(root.right != null){
            path.add(root.right.val);
            helper(root.right, sum + root.right.val, target, path, res);
            path.remove(path.size() - 1);
        }
    }