257. Binary Tree Paths

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> res=new ArrayList<String>();
        StringBuilder save=new StringBuilder();
        dps(root,res,save);
        return res;
    }
    public void dps(TreeNode root, List<String> res, StringBuilder sb)
    {
        if(root==null)
        {
            return;
        }
        int length=sb.length();
        if(root.left==null&&root.right==null)
        {
            sb.append(root.val);
            res.add(sb.toString());
            sb.setLength(length);
            return;
        }
        sb.append(root.val);
        sb.append("->");
        dps(root.left,res,sb);
        dps(root.right,res,sb);
        sb.setLength(length);
    }
        
    
}