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) {
        String path="";
        List<String> result=new ArrayList<String>();
        if(root==null) 
        return result;
        helper(path,root,result);
       
        return result;
    }
    private void helper(String path,TreeNode root,List<String>result){
       
  
        if(root.left==null&&root.right==null){
            path+=""+root.val;
            result.add(path);
            return;
        }
        if(root.left!=null)  
        {path+=""+root.val+"->";
         helper(path,root.left,result);
        }
        if(root.right!=null){
            path+=""+root.val+"->";//path因为是传过来的实参，在这个函数中是全局变量。
            //所以你想当于把root.val->加了两次,对于root左右节点都不是null的。
            helper(path,root.right,result);
        }    
        
        
        
    }
}
正确答案
/**
 * 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) {
        String path="";
        List<String> result=new ArrayList<String>();
        if(root==null) 
        return result;
        helper(path,root,result);
       
        return result;
    }
    private void helper(String path,TreeNode root,List<String>result){
       
  
        if(root.left==null&&root.right==null){
            path+=""+root.val;
            result.add(path);
            return;
        }
        path+=""+root.val+"->";
        if(root.left!=null)  
        {
         helper(path,root.left,result);
        }
        if(root.right!=null){
          
            helper(path,root.right,result);
        }    
        
        
        
    }
}