【C++刷题】优选算法—

【C++刷题】优选算法——分治

颜色分类

void sortColors(vector<int>& nums)
{int left = -1, right = nums.size();for (int i = 0; i < right;) {if (nums[i] == 0) {swap(nums[++left], nums[i++]);} else if (nums[i] == 2) {swap(nums[--right], nums[i]);} else {++i;}}
}

排序数组

// 快速排序
void quick_sort(vector<int>& nums, int start, int end) {if (start >= end) return;int key = nums[rand() % (end - start + 1) + start];int left = start - 1, right = end + 1;for (int i = start; i < right;) {if (nums[i] < key) {swap(nums[++left], nums[i++]);} else if(nums[i] > key) {swap(nums[--right], nums[i]);} else {++i;}}quick_sort(nums, start, left);quick_sort(nums, right, end);
}
vector<int> sortArray(vector<int>& nums) {srand(time(nullptr));quick_sort(nums, 0, nums.size() - 1);return nums;
}// 归并排序
vector<int> tmp;
void merge_sort(vector<int>& nums, int start, int end) {if (start >= end) return;int mid = start + (end - start) / 2;merge_sort(nums, start, mid);merge_sort(nums, mid + 1, end);int left = start, right = mid + 1, i = start;while (left <= mid && right <= end) {if (nums[left] < nums[right]) {tmp[i++] = nums[left++];} else {tmp[i++] = nums[right++];}}while (left <= mid) {tmp[i++] = nums[left++];}while (right <= end) {tmp[i++] = nums[right++];}for (int i = start; i <= end; ++i) {nums[i] = tmp[i];}
}vector<int> sortArray(vector<int>& nums) {tmp = vector<int>(nums.size());merge_sort(nums, 0, nums.size() - 1);return nums;
}

数组中的第K个最大元素

void quick_sort(vector<int>& nums, int start, int end, int k) {if (start >= end) return;int key = nums[rand() % (end - start + 1) + start];int left = start - 1, right = end + 1;for (int i = start; i < right;) {if (nums[i] > key) {swap(nums[++left], nums[i++]);} else if(nums[i] < key) {swap(nums[--right], nums[i]);} else {++i;}}int a = left + 1, b = right - a;if (a >= k) {quick_sort(nums, start, left, k);} else if (a + b >= k) {return;} else {quick_sort(nums, right, end, k);}
}
int findKthLargest(vector<int>& nums, int k) {srand(time(nullptr));quick_sort(nums, 0, nums.size() - 1, k);return nums[k - 1];
}

库存管理 III

void quick_sort(vector<int>& stock, int start, int end, int cnt) {if (start >= end) return;int key = stock[rand() % (end - start + 1) + start];int left = start - 1, right = end + 1;for (int i = start; i < right;) {if (stock[i] < key) {swap(stock[++left], stock[i++]);} else if (stock[i] > key) {swap(stock[--right], stock[i]);} else {++i;}}int a = left + 1, b = right - a;if (a >= cnt) {quick_sort(stock, start, left, cnt);} else if (a + b >= cnt) {return;} else {quick_sort(stock, right, end, cnt);}
}
vector<int> inventoryManagement(vector<int>& stock, int cnt) {srand(time(nullptr));quick_sort(stock, 0, stock.size() - 1, cnt);return vector<int>(stock.begin(), stock.begin() + cnt);
}

交易逆序对的总数

int count = 0;
vector<int> tmp;
void merge_sort(vector<int>& record, int start, int end) {if (start >= end) return;int mid = start + (end - start) / 2;merge_sort(record, start, mid);merge_sort(record, mid + 1, end);int left = start, right = mid + 1, i = start;while (left <= mid && right <= end) {if (record[left] > record[right]) {tmp[i++] = record[left++];count += (end - right + 1); // 灵魂之解} else {tmp[i++] = record[right++];}}while (left <= mid) {tmp[i++] = record[left++];}while (right <= end) {tmp[i++] = record[right++];}for (int i = start; i <= end; ++i) {record[i] = tmp[i];}
}
int reversePairs(vector<int>& record) {tmp.resize(record.size());merge_sort(record, 0, record.size() - 1);return count;
}

计算右侧小于当前元素的个数

vector<int> ret, index, tmp_index, tmp_nums;
void merge_sort(vector<int>& nums, int start, int end) {if (start >= end) return;int mid = start + (end - start) / 2;merge_sort(nums, start, mid);merge_sort(nums, mid + 1, end);int left = start, right = mid + 1, i = start;while (left <= mid && right <= end) {if (nums[left] > nums[right]) {tmp_nums[i] = nums[left];tmp_index[i] = index[left];ret[index[left]] += (end - right + 1);++i;++left;} else {tmp_nums[i] = nums[right];tmp_index[i] = index[right];++i;++right;}}while (left <= mid) {tmp_nums[i] = nums[left];tmp_index[i] = index[left];++i;++left;}while (right <= end) {tmp_nums[i] = nums[right];tmp_index[i] = index[right];++i;++right;}for (int i = start; i <= end; ++i) {nums[i] = tmp_nums[i];index[i] = tmp_index[i];}
}
vector<int> countSmaller(vector<int>& nums) {int n = nums.size();index.resize(n);for (int i = 0; i < n; ++i) {index[i] = i;}ret.resize(n);tmp_nums.resize(n);tmp_index.resize(n);merge_sort(nums, 0, nums.size() - 1);return ret;
}

翻转对

int count = 0;
vector<int> tmp;
void merge_sort(vector<int>& nums, int start, int end) {if (start >= end) return;int mid = start + (end - start) / 2;merge_sort(nums, start, mid);merge_sort(nums, mid + 1, end);int left = start, right = mid + 1;while (left <= mid && right <= end) {if (nums[left] > 2 * (long long)nums[right]) {count += (end - right + 1);++left;} else {++right;}}left = start, right = mid + 1;int i = start;while (left <= mid && right <= end) {if (nums[left] > nums[right]) {tmp[i++] = nums[left++];} else {tmp[i++] = nums[right++];}}while (left <= mid) tmp[i++] = nums[left++];while (right <= end) tmp[i++] = nums[right++];for (int i = start; i <= end; ++i) {nums[i] = tmp[i];}
}
int reversePairs(vector<int>& nums) {tmp.resize(nums.size());merge_sort(nums, 0, nums.size() - 1);return count;
}