257. Binary Tree Paths

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

For example, given the following binary tree:

   1
 /   \
2     3
 \
  5

All root-to-leaf paths are:

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

Solution:
Resursion.
Base case: if root==null return an empty list.
if root is a leaf, return root.val.
else return root.val+"->" + each binaryTreePaths(root.left)  and root.val+"->" + each binaryTreePaths(root.right)

 public List<String> binaryTreePaths(TreeNode root) {  
     List<String> res=new ArrayList<String>();  
     if(root==null) return res;  
     if(root.left==null && root.right==null){  
       res.add(root.val+"");  
       return res;  
     }  
      List<String> leftNode=binaryTreePaths(root.left);  
     for(String s: leftNode){  
       res.add(root.val+"->"+s);  
     }  
      List<String> rightNode=binaryTreePaths(root.right);  
     for(String s: rightNode){  
       res.add(root.val+"->"+s);  
     }  
     return res;  
   }