Binary Tree Paths

Binary Tree Paths
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"]

1.解題思路

求根節點到葉子節點的路徑集合，採用深度搜索，利用遞歸實現。

2.代碼

public class Solution {
    List<String> res=new ArrayList<String>();
    public List<String> binaryTreePaths(TreeNode root) {
        helper(root,new String());
        return res;
        
    }
    private void helper(TreeNode root,String pre){
        if(root==null)  return;
        if(root.left==null&&root.right==null){
            res.add(pre+root.val);
            return;
        }
        helper(root.left,pre+root.val+"->");
        helper(root.right,pre+root.val+"->");
    }
}