演算法

1function cycleEntryHashSet(next: number[]): number {
2  const seen = new Set<number>();
3  for (let node = next.length ? 0 : -1; node !== -1; node = next[node]) {
4    if (seen.has(node)) return node; // the first repeat
5    seen.add(node);
6  }
7  return -1;
8}
9 
10function cycleEntryFloyd(next: number[]): number {
11  if (next.length === 0) return -1;
12  let slow = 0, fast = 0;
13  do {
14    if (next[fast] === -1 || next[next[fast]] === -1) return -1;
15    slow = next[slow];
16    fast = next[next[fast]];
17  } while (slow !== fast);
18  let p = 0; // head and meeting point are equally far from the entry
19  while (p !== slow) {
20    p = next[p];
21    slow = next[slow];
22  }
23  return p;
24}