[leetcode]257. Binary Tree Paths二叉树路径

Given a binary tree, return all root-to-leaf paths.

Note: A leaf is a node with no children.

Example:

Input:

   1
 /   
2     3
 
  5

Output: ["1->2->5", "1->3"]

Explanation: All root-to-leaf paths are: 1->2->5, 1->3

题目

给定一棵二叉树，求出所有根到叶的路径。

思路

DFS， Very straightforward

代码

class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();
        if(root == null) return result;
        helper(root, result, "");
        return result;
    }
    
    private  void helper (TreeNode root, List<String> result,  String path){
        // leaf node
        if(root.left ==null && root.right ==null) {
           result.add(path + root.val); 
        }
        // dfs
        if(root.left!=null){
            helper(root.left, result, path+root.val+ "->");
        }
        if(root.right!=null){
            helper(root.right, result, path+root.val+"->");
        }     
    } 
}