257. Binary Tree Paths

Given a binary tree, return all root-to-leaf paths.

For example, given the following binary tree:

All root-to-leaf paths are:

["1->2->5", "1->3"]

 1 class Solution {
 2     public List<String> binaryTreePaths(TreeNode root) {
 3         List<String> answer = new ArrayList<String>();
 4         if(root!=null) serBST(root,"",answer);
 5         return answer;
 6     }
 7     private void serBST(TreeNode root,String path,List<String> answer){
 8         if(root.left==null && root.right==null) answer.add(path+root.val);
 9         if(root.left!=null)   serBST(root.left,path+root.val+"->",answer);
10         if(root.right!=null) serBST(root.right,path+root.val+"->",answer);
11     }
12 }