257. 二叉树的所有路径 Java版

class Solution {
    public List binaryTreePaths(TreeNode root) {

        List ret = new ArrayList<>();

        if(root == null)
            return ret;

        solve(root,"",ret);

        return ret;
        
    }

    public void solve(TreeNode root, String cur, List ret){
        if(root == null)
            return;

        cur += root.val;

        if(root.left == null && root.right == null){
            ret.add(cur);
        }else{
            solve(root.left, cur+"->", ret);
            solve(root.right, cur+"->", ret);
        }
    }
}