Example of guava TreeTraversal. Iterations: breadthFirst, depthFirst (pre-order, post-order)

guava-tree-traversal.groovy

import com.google.common.collect.TreeTraverser

// format (id/parentId)
def data = [
        '1/0',
    '2/1',      '3/1',
'4/2',      '5/2',       '6/3',
        '7/5',  '8/5',      '9/6'
]


def tree = TreeTraverser.using({
    parent ->
        data.findAll {
            child -> parent.split('/')[0] == child.split('/')[1]
        }
})

def printClosure = {
    println "${' ' * delegate.i++}$it"
}
i = 0
tree.breadthFirstTraversal('1').each(printClosure).last()

/*
1
 2/1
  3/1
   4/2
    5/2
     6/3
      7/5
       8/5
        9/6
 */

i = 0 //reset closure delegate var
tree.preOrderTraversal('1').each(printClosure)
/*
1
 2/1
  4/2
   5/2
    7/5
     8/5
      3/1
       6/3
        9/6
 */
i = 0
tree.postOrderTraversal('1').each(printClosure)
/*
4/2
 7/5
  8/5
   5/2
    2/1
     9/6
      6/3
       3/1
        1
 */