*Binary Tree Paths

题目：

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"]



代码如下：

    public List<String> binaryTreePaths(TreeNode root) 
    {
        List<String> res = new ArrayList<String>();
        StringBuilder sb = new StringBuilder();
        if(root==null)return res;
        helper(root,res,sb);
        return res;
        
    }
    
    public void helper(TreeNode root, List<String> res, StringBuilder sb)
    {
        if(root.left==null&&root.right==null)
        {
            sb.append(root.val);
            res.add(sb.toString());
            return ;
        }
        
        sb.append(root.val);
        sb.append("->");
        
        if(root.left!=null)
        {
            helper(root.left,res, new StringBuilder(sb)); 
            
        }
        
        if(root.right!=null)
        {
            helper(root.right,res, new StringBuilder(sb));
        }
        
    }

红字部分如果只是用 “sb” 就会出现以下错误的结果：

Submission Result: Wrong AnswerMore Details 

Input:[1,2,3]

Output:["1->2","1->23"]

Expected:["1->2","1->3"]

StringBuilder:

 http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html

 

reference: https://leetcode.com/discuss/52250/binary-tree-paths-dfs-java-solution