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4fc14710b6
* fix: initialise properly res, set properly size of ans * test: add check with empty input * style: use const reference as input type * refactor: add ind_type * style: use reverse interators to b * style: use auto in definition of idx * updating DIRECTORY.md * style: clean-up includes * style: use std::string::size_type in definition of ind_type --------- Co-authored-by: github-actions[bot] <github-actions@users.noreply.github.com> Co-authored-by: David Leal <halfpacho@gmail.com>
89 lines
2.6 KiB
C++
89 lines
2.6 KiB
C++
/**
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* @file
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* @brief Program to find the Longest Palindormic
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* Subsequence of a string
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*
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* @details
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* [Palindrome](https://en.wikipedia.org/wiki/Palindrome) string sequence of
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* characters which reads the same backward as forward
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* [Subsequence](https://en.wikipedia.org/wiki/Subsequence) is a sequence that
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* can be derived from another sequence by deleting some or no elements without
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* changing the order of the remaining elements.
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* @author [Anjali Jha](https://github.com/anjali1903)
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*/
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#include <cassert> /// for assert
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#include <string> /// for std::string
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#include <vector> /// for std::vector
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/**
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* Function that returns the longest palindromic
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* subsequence of a string
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*/
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std::string lps(const std::string& a) {
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const auto b = std::string(a.rbegin(), a.rend());
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const auto m = a.length();
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using ind_type = std::string::size_type;
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std::vector<std::vector<ind_type> > res(m + 1,
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std::vector<ind_type>(m + 1));
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// Finding the length of the longest
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// palindromic subsequence and storing
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// in a 2D array in bottoms-up manner
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for (ind_type i = 0; i <= m; i++) {
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for (ind_type j = 0; j <= m; j++) {
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if (i == 0 || j == 0) {
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res[i][j] = 0;
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} else if (a[i - 1] == b[j - 1]) {
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res[i][j] = res[i - 1][j - 1] + 1;
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} else {
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res[i][j] = std::max(res[i - 1][j], res[i][j - 1]);
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}
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}
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}
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// Length of longest palindromic subsequence
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auto idx = res[m][m];
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// Creating string of index+1 length
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std::string ans(idx, '\0');
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ind_type i = m, j = m;
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// starting from right-most bottom-most corner
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// and storing them one by one in ans
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while (i > 0 && j > 0) {
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// if current characters in a and b are same
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// then it is a part of the ans
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if (a[i - 1] == b[j - 1]) {
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ans[idx - 1] = a[i - 1];
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i--;
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j--;
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idx--;
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}
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// If they are not same, find the larger of the
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// two and move in that direction
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else if (res[i - 1][j] > res[i][j - 1]) {
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i--;
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} else {
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j--;
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}
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}
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return ans;
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}
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/** Test function */
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void test() {
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assert(lps("radar") == "radar");
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assert(lps("abbcbaa") == "abcba");
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assert(lps("bbbab") == "bbbb");
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assert(lps("") == "");
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}
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/**
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* Main Function
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*/
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int main() {
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test(); // execute the tests
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return 0;
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}
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