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finding-3-digit-even-numbers.cpp
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finding-3-digit-even-numbers.cpp
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// Time: O(1) ~ O(n), n is 10^3
// Space: O(1)
class Solution {
public:
vector<int> findEvenNumbers(vector<int>& digits) {
vector<int> cnt(10);
for (const auto& d : digits) {
++cnt[d];
}
vector<int> result, curr;
backtracking(&curr, &cnt, &result);
return result;
}
private:
void backtracking(vector<int> *curr, vector<int> *cnt, vector<int> *result) {
static const int k = 3;
if (size(*curr) == k) {
result->emplace_back(accumulate(cbegin(*curr), cend(*curr), 0,
[](const auto& total, const auto& x) {
return total * 10 + x;
}));
return;
}
for (int i = 0; i < size(*cnt); ++i) {
if ((*cnt)[i] == 0 ||
(empty(*curr) && (i == 0)) ||
(size(*curr) == k - 1 && i % 2 != 0)) {
continue;
}
--(*cnt)[i];
curr->emplace_back(i);
backtracking(curr, cnt, result);
curr->pop_back();
++(*cnt)[i];
}
}
};
// Time: O(n), n is 10^3
// Space: O(1)
class Solution2 {
public:
vector<int> findEvenNumbers(vector<int>& digits) {
unordered_map<int, int> cnt;
for (const auto& d : digits) {
++cnt[d];
}
vector<int> result;
for (int i = 1; i < 10; ++i) {
for (int j = 0; j < 10; ++j) {
for (int k = 0; k < 10; k += 2) {
if (cnt[i] > 0 && cnt[j] > (i == j) && cnt[k] > (i == k) + (j == k)) {
result.emplace_back(100 * i + 10 * j + k);
}
}
}
}
return result;
}
};
// Time: O(1) ~ O(n), n is 10^3
// Space: O(1)
class Solution3 {
private:
struct Node {
int digit, cnt, left, right;
Node(int digit, int cnt, int left, int right) : digit(digit), cnt(cnt), left(left), right(right) {}
};
public:
vector<int> findEvenNumbers(vector<int>& digits) {
unordered_map<int, int> cnt;
for (const auto& d : digits) {
++cnt[d];
}
vector<pair<int, int>> digit_cnt;
for (const auto& [d, c] : cnt) {
digit_cnt.emplace_back(d, c);
}
sort(begin(digit_cnt), end(digit_cnt));
vector<Node> digit_cnt_list;
int i = 0;
digit_cnt_list.emplace_back(0, 0, i - 1, i + 1);
++i;
for (const auto& [d, c] : digit_cnt) {
digit_cnt_list.emplace_back(d, c, i - 1, i + 1);
++i;
}
vector<int> result, curr;
backtracking(&curr, &digit_cnt_list, &result);
return result;
}
private:
void backtracking(vector<int> *curr, vector<Node> *digit_cnt_list, vector<int> *result) {
static const int k = 3;
if (size(*curr) == k) {
result->emplace_back(accumulate(cbegin(*curr), cend(*curr), 0,
[](const auto& total, const auto& x) {
return total * 10 + x;
}));
return;
}
for (int i = (*digit_cnt_list)[0].right; i != size(*digit_cnt_list); i = (*digit_cnt_list)[i].right) {
if ((empty(*curr) && (*digit_cnt_list)[i].digit == 0) ||
(size(*curr) == k - 1 && (*digit_cnt_list)[i].digit % 2 != 0)) {
continue;
}
--(*digit_cnt_list)[i].cnt;
curr->emplace_back((*digit_cnt_list)[i].digit);
if ((*digit_cnt_list)[i].cnt == 0) {
if ((*digit_cnt_list)[i].left >= 0) {
(*digit_cnt_list)[(*digit_cnt_list)[i].left].right = (*digit_cnt_list)[i].right;
}
if ((*digit_cnt_list)[i].right < size(*digit_cnt_list)) {
(*digit_cnt_list)[(*digit_cnt_list)[i].right].left = (*digit_cnt_list)[i].left;
}
}
backtracking(curr, digit_cnt_list, result);
curr->pop_back();
if ((*digit_cnt_list)[i].cnt == 0) {
if ((*digit_cnt_list)[i].left >= 0) {
(*digit_cnt_list)[(*digit_cnt_list)[i].left].right = i;
}
if ((*digit_cnt_list)[i].right < size(*digit_cnt_list)) {
(*digit_cnt_list)[(*digit_cnt_list)[i].right].left = i;
}
}
++(*digit_cnt_list)[i].cnt;
}
}
};
// Time: O(1) ~ O(nlogn), n is 10^3
// Space: O(1)
class Solution4 {
public:
vector<int> findEvenNumbers(vector<int>& digits) {
unordered_map<int, int> cnt;
for (const auto& d : digits) {
++cnt[d];
}
vector<pair<int, int>> digit_cnt;
for (const auto& [d, c] : cnt) {
digit_cnt.emplace_back(d, c);
}
vector<int> result, curr;
backtracking(&curr, &digit_cnt, &result);
sort(begin(result), end(result));
return result;
}
private:
void backtracking(vector<int> *curr, vector<pair<int, int>> *digit_cnt, vector<int> *result) {
static const int k = 3;
if (size(*curr) == k) {
result->emplace_back(accumulate(cbegin(*curr), cend(*curr), 0,
[](const auto& total, const auto& x) {
return total * 10 + x;
}));
return;
}
for (int i = 0; i < size(*digit_cnt); ++i) {
if ((empty(*curr) && (*digit_cnt)[i].first == 0) ||
(size(*curr) == k - 1 && (*digit_cnt)[i].first % 2 != 0)) {
continue;
}
--(*digit_cnt)[i].second;
swap((*digit_cnt)[i], (*digit_cnt).back());
auto [digit, cnt] = (*digit_cnt).back();
curr->emplace_back(digit);
if (cnt == 0) {
digit_cnt->pop_back();
}
backtracking(curr, digit_cnt, result);
curr->pop_back();
if (cnt == 0) {
digit_cnt->emplace_back(digit, cnt);
}
swap((*digit_cnt)[i], (*digit_cnt).back());
++(*digit_cnt)[i].second;
}
}
};