Java Returns a set of Strings which represent vertices' data at the k-th level from the vertex with given data "start".

GraphKthLevelChildren.java

/**
 * Returns a set of Strings which represent vertices' data at the k-th level from
 * the vertex with given data "start". The level is 0 at the given vertex. The level
 * increments by 1 when traversal goes one level deeper. For example, consider a graph
 * where A has children {B,C}, B has children {D}, C has children {A}. Then when this
 * method is called as kthLevelChildren("A", 2), the resulting set would contain
 * {A, D}, since these vertices are 2 levels starting from "A".
 * 
 * Note: the purpose of this method is more for testing, in the context of homework 6.
 * 
 * @param start the data of the parent vertex of interest
 * @param k the k-th level at which the set of children would be obtained
 * @return a set of Strings which represent vertices' data at the k-th level from
 *         the vertex with given data "start".
 *         <p>
 *         Returns null if start == null || k < 0 || this graph does not contain a vertex
 *         with data "start"; Returns an empty set if there is no children at kth level.
 */
public Set<String> kthLevelChildren(String start, int k) {
	if (k < 0 || start == null || !vertexMap.containsKey(start)) {
		return null;
	} else {
		return kthLevelChildren(start, k, new HashSet<String>());
		
	}
}

private Set<String> kthLevelChildren(String start, int k, Set<String> result) {
	if (k == 0) {
		result.add(start);
		return result;
	} else {
		Set<String> neighbors = neighbors(start);
		for (String n : neighbors) {
			Set<String> onePath = kthLevelChildren(n, k-1, result);
			for (String v : onePath) {
				result.add(v);
			}
		}
		return result;
	}
}