leetcode Binary Tree Paths

题目链接

思路：
深度优先+递归回溯

/**
 * 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<String> result;

    public List<String> binaryTreePaths(TreeNode root) {
        helpStringBuilder=new StringBuilder();
        result=new ArrayList<String>();
        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());
    }
}