// recursive DFS to detect cycles
public boolean hasCycle() {
// firstly check if this node is a visited clean node
// this pre-check will avoid repeat search and save time
if (cleanNode) {
return false;
}
// if our curr node is a visited node, then it means we got a back edge in current path, so
// there is a cycle
if (visited) {
return true;
}
// mark visited in current search
visited = true;
// check its neighbor nodes
for (Node node : nodes) {
// recursivly search each next node
if (node.hasCycle()) {
return true;
}
}
// reset node as unvisited to avoid the impact on next search
visited = false;
// if there is no cycle in this node, we will set it as cleanNode
cleanNode = true;
return false;
}
private boolean hasCycle(StringBuilder sb) {
// we need put the root node first. To be convenient we want start DFS from any node, not the
// node
// without prerequisites or with most prerequisites. So we will build the string backward.
// We will start build the string after we reach the bottom (the root node). Therefore, in DFS
// we should use "depends on" relation, where we put root node to the bottom
// cleanNode check first, so we can set and reset "visited" freely
if (cleanNode) return false;
if (visited) return true;
visited = true;
for (Node node : nodes) {
if (node.hasCycle(sb)) return true;
}
cleanNode = true;
visited = false;
sb.append(c);
return false;
}
public boolean canFinish(int numCourses, int[][] prerequisites) {
// create a node list, so we can visit node easier
Node[] nodes = new Node[numCourses];
// initialize each node
for (int i = 0; i < nodes.length; i++) {
nodes[i] = new Node();
}
// update neighbors of each node
for (int[] pair : prerequisites) {
nodes[pair[0]].addNode(nodes[pair[1]]);
}
// scan array to check if there is any cycle
for (Node node : nodes) {
if (node.hasCycle()) return false;
}
return true;
}