A bothersome search function on Java multi tree

there is a multi-tree like this?

abc--xx       abc(have 3 children)
      xv--yg   xv(have 2 children)
          ld    
      xu--ph
          ld   xu(have 2 children)
 

design a search function like: search(String key)

search("ld")
the result will be?(need the path from root to the result node)

abc--xv--ld
      xu--ld
 

There is a TreeNode object (you can add your fields if you need)

    TreeNode{
              string key;
              List<TreeNode> children;
     }
 

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