def lmax(l):
    ans = 0
    for i in l:
        ans = max(ans, i)
    return ans

def nth(l, n):
    x = 0
    for i in l:
        if x == n: return i
        else: x = 1 + x

def nth(l, n):
    ans, x = None, 0
    for i in l:
        if x == n: ans = i
        x = x + 1
    return ans

def nth(l, n):
    ans, x = None, 0
    for i in l:
        ans = i if x == n else ans
        x = x + 1
    return ans

def nth(l, n):
    ans, x = None, 0
    def step(ans, x, i):
        newAns = i if x == n else ans
        newX = x + 1
        return newAns, newX
    for i in l:
        ans, x = step(ans, x, i)
    return ans

let nth_opt l n =
    let step (ans, x) i = (if x = n then Some i else ans, x + 1) in
    List.fold_left step (None, 0), l

def unzip(l):
    lefts, rights = [], []
    for i in l:
        a, b = i
        lefts = lefts + [a]
        rights = rights + [a]
    return lefts, rights

let unzip l =
  let step (lefts, rights) (a, b) = (lefts @ [a], rights @ [b]) in
  List.fold_left step ([], []) l

let unzip l =
  let step (a, b) (lefts, rights) = (a :: lefts, b :: rights) in
  List.fold_right step l ([], [])