A. First Letter to Appear Twice (opens new window)

Iterate over the string and return the first character that appears twice.

DETAILS

def repeatedCharacter(self, S: str) -> str:
        counter = [0] * 26
        for ch in S:
            counter[ord(ch) - ord('a')] += 1
            if counter[ord(ch) - ord('a')] == 2:
                return ch

B. Equal Row and Column Pairs (opens new window)

Collect all rows from A in counter, then find the number of occurrence of each row of A in that counter.

DETAILS

def equalPairs(self, A: List[List[int]]) -> int:
        C = collections.Counter(tuple(a) for a in A)
        return sum(C[tuple(a[::-1])] for a in list(zip(*A[::-1])))

C. Design a Food Rating System (opens new window)

Use sorted container, or priority queue.

DETAILS

from sortedcontainers import SortedList
class FoodRatings:

    def __init__(self, F: List[str], C: List[str], R: List[int]):
        self.F, self.pq = {}, defaultdict(SortedList)
        for f, c, r in zip(F, C, R):
            self.F[f] = (c, r)
            self.pq[c].add((-r, f))

    def changeRating(self, fd: str, nr: int) -> None:
        cu, pr = self.F[fd]
        self.F[fd] = (cu, nr)
        self.pq[cu].remove((-pr, fd))
        self.pq[cu].add((-nr, fd))

    def highestRated(self, cu: str) -> str:
        return self.pq[cu][0][1]

D. Number of Excellent Pairs (opens new window)

Please refer to This LC Discussion (opens new window) for my solutions in English.

DETAILS

def countExcellentPairs(self, A: List[int], k: int) -> int:
        def cb(n):
            ans = 0
            while n:
                ans += n & 1
                n >>= 1
            return ans
        
        ans, C = 0, [0] * 31
        for a in set(A):
            C[cb(a)] += 1

        for i in range(1, 31):
            for j in range(1, 31):
                if i + j >= k:
                    ans += C[i] * C[j]

        return ans