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思路：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {
       private StringBuilder helpStringBuilder;    private List
   
     result;    public List
    
      binaryTreePaths(TreeNode root) {        helpStringBuilder=new StringBuilder();        result=new ArrayList
     
      ();        if(root!=null)        {            help(root);        }        return result;    }    private void help(TreeNode root)    {        int beginlength=helpStringBuilder.length();        helpStringBuilder.append(root.val+"->");        boolean leaf=true;        if(root.left!=null)        {            leaf=false;            help(root.left);        }        if(root.right!=null)        {            leaf=false;            help(root.right);        }        if(leaf)        {            helpStringBuilder.delete(helpStringBuilder.length()-2, helpStringBuilder.length());            result.add(helpStringBuilder.toString());        }        helpStringBuilder.delete(beginlength, helpStringBuilder.length());    }}
     
    
   

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