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longest palindromic subsequence memoization python

Amal Krishna answered on August 24, 2022 Popularity 8/10 Helpfulness 3/10

answer longest palindromic subsequence memoization python

related longest palindromic subsequence python

longest palindromic subsequence memoization python

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def lps(t1,t2,text1,text2,memo):
    if t1 >= len(text1) or t2 >= len(text2):
        return 0
    
    key = str(t1) + '-' + str(t2)
    
    if key in memo:
        return memo[key]

    if text1[t1] == text2[t2]:
        memo[key] = 1+lps(t1+1,t2+1,text1,text2,memo)
        return memo[key]
    
    else:
        memo[key] = max(lps(t1+1,t2,text1,text2,memo),lps(t1,t2+1,text1,text2,memo))
        return memo[key]
class Solution:
    def longestPalindromeSubseq(self, s: str) -> int:
        return lps(0,0,s,s[::-1],{})

Popularity 8/10 Helpfulness 3/10 Language python

Source: Grepper

Tags: memoization python subsequence

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Contributed on Aug 24 2022

Amal Krishna

0 Answers Avg Quality 2/10

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longest palindromic subsequence python

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APPROACH 1:
We can use recursion to solve this question. We will keep two pointers, ‘i’ and ‘j’. 
The ‘i’ pointer will initially point at 0, and the ‘j’ pointer will initially point at ‘str.length()-1’. 
At any recursive call, if the character at ‘i’ and ‘j’ positions are the same, we will call our recursive function with the values ‘ i - 1’ and ‘j - 1’ and add 1 to the answer returned by this recursive call as we met a case for a string to be a palindrome.
In the else part, when the characters at the ‘i’ and ‘j’ positions are not the same, we will make two recursive calls with (i, j-1) and (i-1, j) because the elements at ith and jth position are not contributing into a palindromic sequence, and return the maximum among both of them.

Refer to the below implementation for the above approach.

class Main {
 
    // A utility function to find the max of two given integers.
    static int max(int x, int y) {
        return (x > y) ? x : y;
    }
    
    // Returns the longest palindromic subsequence in the arr array
    static int lps(char arr[], int i, int j) {
        
        // Base Case 1: If only 1 character present
        if (i == j) {
            return 1;
        }
 
        // Base Case 2: If only 2 characters are present and both same
        if (arr[i] == arr[j] && i + 1 == j) {
            return 2;
        }
 
        // If first and the last character matches
        if (arr[i] == arr[j]) {
            return lps(arr, i + 1, j - 1) + 2;
        }
 
        // If first and the last characters does not match
        return max(lps(arr, i, j - 1), lps(arr, i + 1, j));
    }
 
 
    public static void main(String[] args) {
        String seq = "ZXYFYKXAC";
        int n = seq.length();

        System.out.printf("The length of the Longest Palindromic Subsequence is %d", lps(seq.toCharArray(), 0, n - 1));
    }
}

Popularity 9/10 Helpfulness 1/10 Language python

Source: www.codingninjas.com

Tags: subsequence python subsequenc

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Contributed on Oct 13 2022

Prickly Pigeon

0 Answers Avg Quality 2/10

longest palindromic subsequence memoization python

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