Given tree
root = TreeNode ( 20 ) NEW_LINE left = TreeNode ( 12 ) NEW_LINE right = TreeNode ( 18 ) NEW_LINE root . children . append ( left ) NEW_LINE root . children . append ( right ) NEW_LINE left . children . append ( TreeNode ( 11 ) ) NEW_LINE left . children . append ( TreeNode ( 3 ) ) NEW_LINE right . children . append ( TreeNode ( 15 ) ) NEW_LINE right . children . append ( TreeNode ( 8 ) ) NEW_LINE