1、前言

2、思路

思路是按照回溯的方法来写的，本来叶子节点那有一个 list.remove(list.size() - 1) 的判断，但是后面又重复了，所以去掉了，只需要最后加就行。

3、代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {

    List<String> res = new ArrayList<>();

    public List<String> binaryTreePaths(TreeNode root) {
        if(root == null){
            return res;
        }
        
        dfs(root, new ArrayList<>());
        return res;
    }

    private void dfs(TreeNode root, List<Integer> list){
        if(root == null){
            return;
        }

        list.add(root.val);

        if(root.left == null && root.right == null){
            String s = addRes(list);
            res.add(s);
        }

        dfs(root.left, list);
        dfs(root.right, list);
        list.remove(list.size() - 1);
    }

    private String addRes(List<Integer> list){
        String s = "";
        for(int i = 0; i < list.size() - 1; i++){
            s += list.get(i) + "->";
        }
        s += list.get(list.size() - 1);
        return s;
    }
}