257. Binary Tree Paths

题目描述

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"]

解题方法

判別root->left == null

   root->right == null  再加以遞迴

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

    private void DFS(TreeNode root, List<String> list, String str){
        if(root == null){ return;}
        //leaf node
        if(root.left == null && root.right == null){
            str += root.val;
            list.add(str);
            return;
        }

        if(root.left != null){
            DFS(root.left, list, str + root.val + "->");
        }
        if(root.right != null){
            DFS(root.right, list, str + root.val + "->");
        }
    }