1. A method for generating an array of node identifiers for respective vertex nodes in a computer having data in the form of a tree data structure, in which nodes are assigned their respective unique node identifiers and a parent-child relationship is represented by a first array containing the node identifiers assigned to parent nodes, said node identifiers assigned to the parent nodes being associated with the node identifiers assigned to non-root nodes that are nodes other than a root node, the method comprising the steps of: providing a second array containing the node identifiers assigned to respective vertex nodes in order to represent one or more node groups, wherein each node group includes a particular node, which is the vertex node, and descendant nodes of the particular node; and deleting the node identifier from the second array if said node identifier in the second array is identical to another node identifier in the second array or the descendant nodes of the node identified by said another node identifier by referring to the parent-child relationship represented by the first array, wherein the step of deleting the node identifiers further comprises the steps of: generating a third array whose size is the same as that of the first array; and deleting from the second array one of two node identifiers, including a first node identifier stored in the second array and a second node identifier present in the third array, the first node identifier being a descendant node of the second node identifier, if the second node identifier is stored in the third array at positions, which are specified by the node identifier for the vertex node identified by the first node identifier stored in the second array, and specified by the node identifiers for the respective descendant nodes of said vertex node, by referring to the parent-child relationship represented by the first array, while storing the first node identifier in the third array at said positions.