solution in python it passes all test cases

def ashtonString(s, k): kv = k - 1 N = len(s) sr = [[0 for _ in range(N)] for __ in range(17)] sr[0] = [ord(c)-97 for c in s]

L = [0]*N
cnt = 1
kf = lambda x: x[0]*(N+1) + x[1]
for i in range(1, 17):
    for j in range(N):
        L[j] = (sr[i-1][j], sr[i-1][j+cnt] if j+cnt < N else -1, j)
    L.sort(key=kf)

    sr[i][L[0][2]] = 0
    cr = 0
    for j in range(1, N):
        if L[j-1][0] != L[j][0] or L[j-1][1] != L[j][1]:
            cr += 1
        sr[i][L[j][2]] = cr
    cnt *= 2
    if cnt >= N:
        break

sa = [l[2] for l in L]
rank = [0]*N
lcp = [0]*N

for i in range(N):
    rank[sa[i]] = i

k = 0
for i in range(N):
    if rank[i] == N-1:
        k = 0
        continue
    j = sa[rank[i] + 1]
    while j + k < N and i + k < N and s[i+k] == s[j+k]:
        k += 1
    lcp[rank[i]] = k
    if k > 0:
        k -= 1

numprev = 0
tri = lambda x: ((x+1)*x)>>1
print(sa)
print(lcp)
for i in range(N):
    mylen = N - sa[i]
    tt = tri(mylen) - tri(numprev)
    if kv < tt:
        for j in range(1 + numprev, 1 + mylen):
            if kv < j:
                return s[sa[i]+kv]
            kv -= j
    kv -= tt
    numprev = lcp[i]

return ''