给个数组，打乱了，比如：索引 0 1 2 3 4；值 3 2 1 4 0 。数组的值是下次跳的索引位置，这样的话数组有环，比如 0 -> 3 -> 4 -> 0 1 -> 2 -> 1， 求最长环的长度。

a.ml

let huangBingchao a =
  let n = Array.length a in
  let rec go2 j i l =
    if a.(i) < 0 then
      (j, l)
    else
      let ii = a.(i) in
      a.(i) <- j;
      go2 (lnot i) ii (l+1) in
  let rec go i mx =
    if i < 0 then
      mx
    else
      let (ix, l) = if a.(i) < 0 then (a.(i), 0) else go2 (-1) i 0 in
      a.(i) <- lnot ix;
      go (i-1) (max mx l) in
  go (n-1) 0

let read_int () = Scanf.scanf " %d" (fun x -> x)
let n = read_int()
let a = Array.init n (fun _ -> read_int());;
Printf.printf "%d\n" (huangBingchao a);