Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque


A = input()
N = len(A)
a = ord('a')

edge = [[N + 1] * 26 for _ in range(N + 1)]
for i in range(N - 1, -1, -1):
    for j in range(26):
        edge[i][j] = edge[i + 1][j]
    c = ord(A[i]) - a
    edge[i][c] = i + 1


recon = [None] * (N + 2)  # DP復元用。(直前の頂点, 文字)を格納する。
q = deque()
q.append(0)  # 頂点
while q:
    i = q.popleft()
    if i == N + 1:  # 頂点N+1に到達したら処理終了
        break
    for j in range(26):
        ni = edge[i][j]
        if recon[ni] is None:  # その頂点に最短で到達した場合のみ次の処理を継続
            recon[ni] = (i, chr(a + j))
            q.append(ni)

i = N + 1
ans = []
while i > 0:
    pi, c = recon[i]
    ans.append(c)
    i = pi

print(''.join(reversed(ans)))


Fixed submission:
def main():
    # import sys
    # readline = sys.stdin.readline
    # readlines = sys.stdin.readlines
    
    A = input()
    N = len(A)
    a = ord('a')

    INF = N
    na = [[INF] * 26 for _ in range(N + 1)]
    for i in range(N - 1, -1, -1):
        c = ord(A[i]) - a
        for j in range(26):
            na[i][j] = na[i + 1][j]
        na[i][c] = i

    dp = [INF] * (N + 1)
    dp[N] = 1
    recon = [None] * (N + 1)
    for i in range(N - 1, -1, -1):
        for j in range(26):
            ni = na[i][j]
            if ni == N:
                if dp[i] > 1:
                    dp[i] = 1
                    recon[i] = (chr(a + j), N)
            elif dp[i] > dp[ni + 1] + 1:
                dp[i] = dp[ni + 1] + 1
                recon[i] = (chr(a + j), ni + 1)

    # k =
    # for j in range(26):
    #     ni = na[0][j]
    #     if dp[i] > dp[ni] + 1:

    i = 0
    ans = []
    while i < N:
        c, ni = recon[i]
        ans.append(c)
        i = ni

    print(''.join(ans))


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    # import sys
    # readline = sys.stdin.readline
    # readlines = sys.stdin.readlines
    from collections import deque

    A = input()
    N = len(A)
    a = ord('a')

    INF = N
    na = [[INF] * 26 for _ in range(N)]
    for i in range(N - 1, -1, -1):
        c = ord(A[i]) - ord('a')
        for j in range(26):
            na[i][j] = i if c == j else na[(i + 1) % N][j]

    q = deque()
    q.append(([], -1))

    while q:
        s, i = q.popleft()
        for j in range(26):
            ni = na[i + 1][j]
            ns = s + [chr(a + j)]
            if ni == INF:
                print(''.join(ns))
                exit()
            else:
                q.append((ns, ni))


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    # import sys
    # readline = sys.stdin.readline
    # readlines = sys.stdin.readlines
    
    A = input()
    N = len(A)
    a = ord('a')

    INF = N
    na = [[INF] * 26 for _ in range(N + 1)]
    for i in range(N - 1, -1, -1):
        c = ord(A[i]) - a
        for j in range(26):
            na[i][j] = na[i + 1][j]
        na[i][c] = i

    dp = [INF] * (N + 1)
    dp[N] = 1
    recon = [None] * (N + 1)
    for i in range(N - 1, -1, -1):
        for j in range(26):
            ni = na[i][j]
            if ni == N:
                if dp[i] > 1:
                    dp[i] = 1
                    recon[i] = (chr(a + j), N)
            elif dp[i] > dp[ni + 1] + 1:
                dp[i] = dp[ni + 1] + 1
                recon[i] = (chr(a + j), ni + 1)

    # k =
    # for j in range(26):
    #     ni = na[0][j]
    #     if dp[i] > dp[ni] + 1:

    i = 0
    ans = []
    while i < N:
        c, ni = recon[i]
        ans.append(c)
        i = ni

    print(''.join(ans))


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque


A = input()
N = len(A)
a = ord('a')

INF = N
na = [[INF] * 26 for _ in range(N)]
for i in range(N - 1, -1, -1):
    c = ord(A[i]) - ord('a')
    for j in range(26):
        na[i][j] = i if c == j else na[(i + 1) % N][j]

q = deque()
q.append(('', -1))

while q:
    s, i = q.popleft()
    for j in range(26):
        ni = na[i + 1][j]
        if ni == INF:
            print(s + chr(a + j))
            exit()
        else:
            q.append((s + chr(a + j), ni))


Fixed submission:
def main():
    # import sys
    # readline = sys.stdin.readline
    # readlines = sys.stdin.readlines
    
    A = input()
    N = len(A)
    a = ord('a')

    INF = N
    na = [[INF] * 26 for _ in range(N + 1)]
    for i in range(N - 1, -1, -1):
        c = ord(A[i]) - a
        for j in range(26):
            na[i][j] = na[i + 1][j]
        na[i][c] = i

    dp = [INF] * (N + 1)
    dp[N] = 1
    recon = [None] * (N + 1)
    for i in range(N - 1, -1, -1):
        for j in range(26):
            ni = na[i][j]
            if ni == N:
                if dp[i] > 1:
                    dp[i] = 1
                    recon[i] = (chr(a + j), N)
            elif dp[i] > dp[ni + 1] + 1:
                dp[i] = dp[ni + 1] + 1
                recon[i] = (chr(a + j), ni + 1)

    # k =
    # for j in range(26):
    #     ni = na[0][j]
    #     if dp[i] > dp[ni] + 1:

    i = 0
    ans = []
    while i < N:
        c, ni = recon[i]
        ans.append(c)
        i = ni

    print(''.join(ans))


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque


A = input()
N = len(A)
a = ord('a')

INF = N
na = [[INF] * 26 for _ in range(N)]
for i in range(N - 1, -1, -1):
    c = ord(A[i]) - ord('a')
    for j in range(26):
        na[i][j] = i if c == j else na[(i + 1) % N][j]

q = deque()
q.append(('', -1))

while q:
    s, i = q.popleft()
    for j in range(26):
        ni = na[i + 1][j]
        if ni == INF:
            print(s + chr(a + j))
            exit()
        else:
            q.append((s + chr(a + j), ni))


Fixed submission:
def main():
    # import sys
    # readline = sys.stdin.readline
    # readlines = sys.stdin.readlines
    
    A = input()
    N = len(A)
    a = ord('a')

    INF = N
    na = [[INF] * 26 for _ in range(N + 1)]
    for i in range(N - 1, -1, -1):
        c = ord(A[i]) - a
        for j in range(26):
            na[i][j] = na[i + 1][j]
        na[i][c] = i

    dp = [INF] * (N + 1)
    dp[N] = 1
    recon = [None] * (N + 1)
    for i in range(N - 1, -1, -1):
        for j in range(26):
            ni = na[i][j]
            if ni == N:
                if dp[i] > 1:
                    dp[i] = 1
                    recon[i] = (chr(a + j), N)
            elif dp[i] > dp[ni + 1] + 1:
                dp[i] = dp[ni + 1] + 1
                recon[i] = (chr(a + j), ni + 1)

    # k =
    # for j in range(26):
    #     ni = na[0][j]
    #     if dp[i] > dp[ni] + 1:

    i = 0
    ans = []
    while i < N:
        c, ni = recon[i]
        ans.append(c)
        i = ni

    print(''.join(ans))


if __name__ == "__main__":
    main()