Matter++

import sys
input = sys.stdin.readline

def to_num(c):
    return ord(c) - ord('a')


w = int(input())
s = input().strip()
n = len(s)
cur_w = 0


frequency = [[0] * 26 for _ in range(n + 1)]
# ww = [0] * n
# ww[0] = 1
for i in range(1, n + 1):
    frequency[i] = frequency[i - 1].copy()
    frequency[i][to_num(s[i - 1])] += 1
    # if i != 0:
    #     ww[i] = ww[i - 1] + 2 * frequency[i][to_num(s[i])] - 1
# print(frequency)
left_s = -1
right_s = n
left = 0
right = -1
cur_w = 0
while right < n - 1 or cur_w > w:
    if cur_w > w:
        left += 1
    else:
        right += 1
    cur_w = 0
    # print(left, right)
    for i in range(26):
        cur_w += (frequency[right + 1][i] - frequency[left][i]) ** 2
    # print(cur_w)
    if cur_w >= w and right - left < right_s - left_s:
        # print('here')
        left_s = left
        right_s = right

print(s[left_s:right_s + 1])

Test 1

Verdict: ACCEPTED

Test 2

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Test 68

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Test 69

Verdict: ACCEPTED

Test 70

Verdict: ACCEPTED

Test 71

Verdict: ACCEPTED