mirror of
https://github.com/TheAlgorithms/C-Plus-Plus.git
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137 lines
4.2 KiB
C++
137 lines
4.2 KiB
C++
/**
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* @file
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* @brief Implementation of [Sparse
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* Table](https://brilliant.org/wiki/sparse-table/) for `min()` function.
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* @author [Mann Patel](https://github.com/manncodes)
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* @details
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* Sparse Table is a data structure, that allows answering range queries.
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* It can answer most range queries in O(logn), but its true power is answering
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* range minimum queries (or equivalent range maximum queries). For those
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* queries it can compute the answer in O(1) time. The only drawback of this
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* data structure is, that it can only be used on immutable arrays. This means,
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* that the array cannot be changed between two queries.
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*
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* If any element in the array changes, the complete data structure has to be
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* recomputed.
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*
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* @warning
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* This sparse table is made for `min(a1,a2,...an)` duplicate invariant
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* function. This implementation can be changed to other functions like
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* `gcd()`, `lcm()`, and `max()` by changing a few lines of code.
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*/
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#include <array> /// for std::array
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#include <cassert> /// for assert
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#include <iostream> /// for IO operations
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/**
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* @namespace data_structures
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* @brief Data Structures algorithms
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*/
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namespace data_structures {
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/**
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* @namespace sparse_table
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* @brief sparse table and it's util functions.
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*/
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namespace sparse_table {
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/**
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* @brief A struct to represent sparse table for `min()` as their invariant
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* function, for the given array `A`. The answer to queries are stored in array
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* ST.
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*/
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struct Sparse_table {
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const static int N = 12345; ///< the maximum size of the array.
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const static int M = 14; ///< ceil(log2(N)).
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int n; ///< size of input array.
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/** @warning check if `N` is not less than `n`. if so, manually increase the
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* value of N */
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std::array<int, N> A; ///< input array to perform RMQ.
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std::array<std::array<int, N>, M>
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ST; ///< the sparse table storing `min()` values for given interval.
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std::array<int, N> LOG; ///< where floor(log2(i)) are precomputed.
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/**
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* @brief Builds the sparse table for computing min/max/gcd/lcm/...etc
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* for any contiguous sub-segment of the array.This is an example of
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* computing the index of the minimum value.
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* @return void
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* @complexity: O(n.log(n))
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*/
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void buildST() {
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LOG[0] = -1;
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for (int i = 0; i < n; ++i) {
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ST[0][i] = i;
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LOG[i + 1] = LOG[i] + !(i & (i + 1));
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}
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for (int j = 1; (1 << j) <= n; ++j) {
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for (int i = 0; (i + (1 << j)) <= n; ++i) {
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int x = ST[j - 1][i];
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int y = ST[j - 1][i + (1 << (j - 1))];
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ST[j][i] = (A[x] <= A[y] ? x : y);
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}
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}
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}
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/**
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* @brief Queries the sparse table for the value of the interval [l, r]
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* (i.e. from l to r inclusive).
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*
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* @complexity: O(1)
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*
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* @param l the left index of the range (inclusive).
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* @param r the right index of the range (inclusive).
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*
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* @return the computed value of the given interval.
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*/
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int query(int l, int r) {
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int g = LOG[r - l + 1];
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int x = ST[g][l];
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int y = ST[g][r - (1 << g) + 1];
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return (A[x] <= A[y] ? x : y);
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}
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};
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} // namespace sparse_table
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} // namespace data_structures
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/**
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* @brief Self-test implementations
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* @returns void
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*/
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static void test() {
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std::array<int, 10> testcase = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
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int testcase_size = sizeof(testcase) / sizeof(testcase[0]);
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data_structures::sparse_table::Sparse_table st{};
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std::copy(std::begin(testcase), std::end(testcase),
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std::begin(st.A)); ///< copying testcase to the struct
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st.n = testcase_size;
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st.buildST(); ///< precomputing sparse tree
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// pass queries of the form: [l,r]
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assert(st.query(1, 9) == 1); ///< as 1 is smallest from 1..9
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assert(st.query(2, 6) == 2); ///< as 2 is smallest from 2..6
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assert(st.query(3, 8) == 3); ///< as 3 is smallest from 3..8
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std::cout << "Testcase passed!" << std::endl;
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}
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/**
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* @brief Main function
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* @param argc commandline argument count (ignored)
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* @param argv commandline array of arguments (ignored)
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* @returns 0 on exit
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*/
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int main(int argc, char *argv[]) {
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test(); // run self-test implementations
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return 0;
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}
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