## Online Majority Element In Subarray

• Time:Constructor: O(n), query(left: int, right: int, threshold: int): O(\log n)
• Space:O(n)

## C++

``````class MajorityChecker {
public:
MajorityChecker(vector<int>& arr) : A(arr) {
for (int i = 0; i < A.size(); ++i)
numToIndices[A[i]].push_back(i);
}

int query(int left, int right, int threshold) {
for (int i = 0; i < kTimes; ++i) {
const int num = A[rand() % A.size()];
const auto& indices = numToIndices[num];
const auto lit = lower_bound(begin(indices), end(indices), left);
const auto rit = upper_bound(begin(indices), end(indices), right);
if (rit - lit >= threshold)
return num;
}

return -1;
}

private:
const vector<int> A;
constexpr static int kTimes = 20;  // 2^kTimes >> A.size() = n
unordered_map<int, vector<int>> numToIndices;
};
``````

## JAVA

``````class MajorityChecker {
public MajorityChecker(int[] arr) {
A = arr;
for (int i = 0; i < A.length; ++i) {
if (!numToIndices.containsKey(A[i]))
numToIndices.put(A[i], new ArrayList<>());
numToIndices.get(A[i]).add(i);
}
}

public int query(int left, int right, int threshold) {
for (int i = 0; i < kTimes; ++i) {
final int num = A[rand.nextInt(A.length)];
List<Integer> indices = numToIndices.get(num);
final int l = firstGreaterEqual(indices, left);
final int r = firstGreaterEqual(indices, right + 1);
if (r - l >= threshold)
return num;
}

return -1;
}

private static final int kTimes = 20; // 2^kTimes >> A.length
private int[] A;
private Map<Integer, List<Integer>> numToIndices = new HashMap<>();
private Random rand = new Random();

private int firstGreaterEqual(List<Integer> indices, int target) {
int index = Collections.binarySearch(indices, target);
return index < 0 ? -index - 1 : index;
}
}
``````

## Python

``````class MajorityChecker:
def __init__(self, arr: List[int]):
self.A = arr
self.kTimes = 20  # 2^kTimes >> len(A)
self.numToIndices = defaultdict(list)

for i, a in enumerate(self.A):
self.numToIndices[a].append(i)

def query(self, left: int, right: int, threshold: int) -> int:
for _ in range(self.kTimes):
randIndex = randint(0, len(self.A) - 1)
num = self.A[randIndex]
indices = self.numToIndices[num]
l = bisect.bisect_left(indices, left)
r = bisect.bisect_right(indices, right)
if r - l >= threshold:
return num

return -1
``````