Given two strings of lower alphabet characters, we need to find the number of ways to insert a character in the first string such that length of LCS of both strings increases by one.
Examples:
Input : str1 = “abab”, str2 = “abc” Output : 3 LCS length of given two strings is 2. There are 3 ways of insertion in str1, to increase the LCS length by one which are enumerated below, str1 = “abcab” str2 = “abc” LCS length = 3 str1 = “abacb” str2 = “abc” LCS length = 3 str1 = “ababc” str2 = “abc” LCS length = 3 Input : str1 = “abcabc”, str2 = “abcd” Output : 4
The idea is try all 26 possible characters at each position of first string, if length of str1 is m then a new character can be inserted in (m + 1) positions, now suppose at any time character c is inserted at ith position in str1 then we will match it with all positions having character c in str2. Suppose one such position is j, then for total LCS length to be one more than previous, below condition should satisfy,
LCS(str1[1, m], str2[1, n]) = LCS(str1[1, i], str2[1, j-1]) +
LCS(str1[i+1, m], str2[j+1, n])
Above equation states that sum of LCS of the suffix and prefix substrings at inserted character must be same as total LCS of strings, so that when the same character is inserted in first string it will increase the length of LCS by one.
In below code two 2D arrays, lcsl and lcsr are used for storing LCS of prefix and suffix of strings respectively. Method for filling these 2D arrays can be found here.
Please see below code for better understanding,
C++
// C++ program to get number of ways to increase// LCS by 1#include <bits/stdc++.h>using namespace std;#define M 26// Utility method to get integer position of lower// alphabet characterint toInt(char ch){ return (ch - 'a');}// Method returns total ways to increase LCS length by 1int waysToIncreaseLCSBy1(string str1, string str2){ int m = str1.length(), n = str2.length(); // Fill positions of each character in vector vector<int> position[M]; for (int i = 1; i <= n; i++) position[toInt(str2[i-1])].push_back(i); int lcsl[m + 2][n + 2]; int lcsr[m + 2][n + 2]; // Initializing 2D array by 0 values for (int i = 0; i <= m+1; i++) for (int j = 0; j <= n + 1; j++) lcsl[i][j] = lcsr[i][j] = 0; // Filling LCS array for prefix substrings for (int i = 1; i <= m; i++) { for (int j = 1; j <= n; j++) { if (str1[i-1] == str2[j-1]) lcsl[i][j] = 1 + lcsl[i-1][j-1]; else lcsl[i][j] = max(lcsl[i-1][j], lcsl[i][j-1]); } } // Filling LCS array for suffix substrings for (int i = m; i >= 1; i--) { for (int j = n; j >= 1; j--) { if (str1[i-1] == str2[j-1]) lcsr[i][j] = 1 + lcsr[i+1][j+1]; else lcsr[i][j] = max(lcsr[i+1][j], lcsr[i][j+1]); } } // Looping for all possible insertion positions // in first string int ways = 0; for (int i=0; i<=m; i++) { // Trying all possible lower case characters for (char c='a'; c<='z'; c++) { // Now for each character, loop over same // character positions in second string for (int j=0; j<position[toInt(c)].size(); j++) { int p = position[toInt(c)][j]; // If both, left and right substrings make // total LCS then increase result by 1 if (lcsl[i][p-1] + lcsr[i+1][p+1] == lcsl[m][n]) ways++; } } } return ways;}// Driver code to test above methodsint main(){ string str1 = "abcabc"; string str2 = "abcd"; cout << waysToIncreaseLCSBy1(str1, str2); return 0;} |
Java
// Java program to get number of ways to increase // LCS by 1import java.util.*;class GFG { static int M = 26; // Method returns total ways to increase // LCS length by 1 static int waysToIncreaseLCSBy1(String str1, String str2) { int m = str1.length(), n = str2.length(); // Fill positions of each character in vector Vector<Integer>[] position = new Vector[M]; for (int i = 0; i < M; i++) position[i] = new Vector<>(); for (int i = 1; i <= n; i++) position[str2.charAt(i - 1) - 'a'].add(i); int[][] lcsl = new int[m + 2][n + 2]; int[][] lcsr = new int[m + 2][n + 2]; // Initializing 2D array by 0 values for (int i = 0; i <= m + 1; i++) for (int j = 0; j <= n + 1; j++) lcsl[i][j] = lcsr[i][j] = 0; // Filling LCS array for prefix substrings for (int i = 1; i <= m; i++) { for (int j = 1; j <= n; j++) { if (str1.charAt(i - 1) == str2.charAt(j - 1)) lcsl[i][j] = 1 + lcsl[i - 1][j - 1]; else lcsl[i][j] = Math.max(lcsl[i - 1][j], lcsl[i][j - 1]); } } // Filling LCS array for suffix substrings for (int i = m; i >= 1; i--) { for (int j = n; j >= 1; j--) { if (str1.charAt(i - 1) == str2.charAt(j - 1)) lcsr[i][j] = 1 + lcsr[i + 1][j + 1]; else lcsr[i][j] = Math.max(lcsr[i + 1][j], lcsr[i][j + 1]); } } // Looping for all possible insertion positions // in first string int ways = 0; for (int i = 0; i <= m; i++) { // Trying all possible lower case characters for (char d = 0; d < 26; d++) { // Now for each character, loop over same // character positions in second string for (int j = 0; j < position[d].size(); j++) { int p = position[d].elementAt(j); // If both, left and right substrings make // total LCS then increase result by 1 if (lcsl[i][p - 1] + lcsr[i + 1][p + 1] == lcsl[m][n]) ways++; } } } return ways; } // Driver Code public static void main(String[] args) { String str1 = "abcabc"; String str2 = "abcd"; System.out.println(waysToIncreaseLCSBy1(str1, str2)); }}// This code is contributed by// sanjeev2552 |
Python3
# Python3 program to get number of ways to increase# LCS by 1M = 26# Method returns total ways to increase LCS length by 1def waysToIncreaseLCSBy1(str1, str2): m = len(str1) n = len(str2) # Fill positions of each character in vector # vector<int> position[M]; position = [[] for i in range(M)] for i in range(1, n+1, 1): position[ord(str2[i-1])-97].append(i) # Initializing 2D array by 0 values lcsl = [[0 for i in range(n+2)] for j in range(m+2)] lcsr = [[0 for i in range(n+2)] for j in range(m+2)] # Filling LCS array for prefix substrings for i in range(1, m+1, 1): for j in range(1, n+1,1): if (str1[i-1] == str2[j-1]): lcsl[i][j] = 1 + lcsl[i-1][j-1] else: lcsl[i][j] = max(lcsl[i-1][j], lcsl[i][j-1]) # Filling LCS array for suffix substrings for i in range(m, 0, -1): for j in range(n, 0, -1): if (str1[i-1] == str2[j-1]): lcsr[i][j] = 1 + lcsr[i+1][j+1] else: lcsr[i][j] = max(lcsr[i+1][j], lcsr[i][j+1]) # Looping for all possible insertion positions # in first string ways = 0 for i in range(0, m+1,1): # Trying all possible lower case characters for C in range(0, 26,1): # Now for each character, loop over same # character positions in second string for j in range(0, len(position[C]),1): p = position[C][j] # If both, left and right substrings make # total LCS then increase result by 1 if (lcsl[i][p-1] + lcsr[i+1][p+1] == lcsl[m][n]): ways += 1 return ways# Driver code to test above methodsstr1 = "abcabc"str2 = "abcd"print(waysToIncreaseLCSBy1(str1, str2))# This code is contributed by ankush_953 |
C#
// C# program to get number of ways// to increase LCS by 1using System;using System.Collections.Generic;class GFG{ static int M = 26;// Method returns total ways to increase // LCS length by 1static int waysToIncreaseLCSBy1(String str1, String str2) { int m = str1.Length, n = str2.Length; // Fill positions of each character in vector List<int>[] position = new List<int>[M]; for(int i = 0; i < M; i++) position[i] = new List<int>(); for(int i = 1; i <= n; i++) position[str2[i - 1] - 'a'].Add(i); int[,] lcsl = new int[m + 2, n + 2]; int[,] lcsr = new int[m + 2, n + 2]; // Initializing 2D array by 0 values for(int i = 0; i <= m + 1; i++) for(int j = 0; j <= n + 1; j++) lcsl[i, j] = lcsr[i, j] = 0; // Filling LCS array for prefix substrings for(int i = 1; i <= m; i++) { for(int j = 1; j <= n; j++) { if (str1[i - 1] == str2[j - 1]) lcsl[i, j] = 1 + lcsl[i - 1, j - 1]; else lcsl[i, j] = Math.Max(lcsl[i - 1, j], lcsl[i, j - 1]); } } // Filling LCS array for suffix substrings for(int i = m; i >= 1; i--) { for(int j = n; j >= 1; j--) { if (str1[i - 1] == str2[j - 1]) lcsr[i, j] = 1 + lcsr[i + 1, j + 1]; else lcsr[i, j] = Math.Max(lcsr[i + 1, j], lcsr[i, j + 1]); } } // Looping for all possible insertion // positions in first string int ways = 0; for(int i = 0; i <= m; i++) { // Trying all possible lower // case characters for(int d = 0; d < 26; d++) { // Now for each character, loop over same // character positions in second string for(int j = 0; j < position[d].Count; j++) { int p = position[d][j]; // If both, left and right substrings make // total LCS then increase result by 1 if (lcsl[i, p - 1] + lcsr[i + 1, p + 1] == lcsl[m, n]) ways++; } } } return ways;}// Driver Codepublic static void Main(String[] args) { String str1 = "abcabc"; String str2 = "abcd"; Console.WriteLine(waysToIncreaseLCSBy1(str1, str2));}}// This code is contributed by Princi Singh |
Javascript
<script>// JavaScript program to get number of ways to increase// LCS by 1let M = 26// Method returns total ways to increase LCS length by 1function waysToIncreaseLCSBy1(str1, str2){ let m = str1.length; let n = str2.length; // Fill positions of each character in vector // vector<int> position[M]; let position = new Array(M); for(let i = 0; i < M; i++) position[i] = []; for(let i = 1; i < n + 1; i++) { position[str2.charCodeAt(i-1)-97].push(i) } // Initializing 2D array by 0 values let lcsl = new Array(m+2); for(let i = 0; i < m + 2; i++) { lcsl[i] = new Array(n+2).fill(0); } let lcsr = new Array(m + 2); for(let i = 0; i < m + 2; i++) { lcsr[i] = new Array(n+2).fill(0); } // Filling LCS array for prefix substrings for(let i = 1; i < m + 1; i++) { for(let j = 1; j < n + 1; j++) { if (str1[i-1] == str2[j-1]) lcsl[i][j] = 1 + lcsl[i-1][j-1] else lcsl[i][j] = Math.max(lcsl[i-1][j], lcsl[i][j-1]) } } // Filling LCS array for suffix substrings for(let i = m; i > 0; i--) { for(let j = n; j > 0; j--) { if (str1[i-1] == str2[j-1]) lcsr[i][j] = 1 + lcsr[i+1][j+1] else lcsr[i][j] = Math.max(lcsr[i+1][j], lcsr[i][j+1]) } } // Looping for all possible insertion positions // in first string let ways = 0 for(let i = 0; i < m + 1; i++) { // Trying all possible lower case characters for(let C = 0; C < 26; C++) { // Now for each character, loop over same // character positions in second string for(let j = 0; j < position[C].length; j++) { let p = position[C][j] // If both, left and right substrings make // total LCS then increase result by 1 if (lcsl[i][p-1] + lcsr[i+1][p+1] == lcsl[m][n]) ways += 1 } } } return ways}// Driver code to test above methodslet str1 = "abcabc"let str2 = "abcd"document.write(waysToIncreaseLCSBy1(str1, str2))// This code is contributed by shinjanpatra</script> |
Output
4
Time Complexity : O(mn)
Auxiliary Space : O(mn)
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