Lesson overview · Free interview practice

Palindromic Subsequence

Identify palindromic subsequences within strings.

Topics in the full lesson
  • Concept and Use Cases
  • When to Use
  • Time and Space Complexity
  • Common Problems
  • Longest Palindromic Subsequence
  • Minimum Deletions to Make a String Palindromic

Practice Problems

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CodingMedium

Longest Palindromic Subsequence

Question

Given a string s, find the longest subsequence that reads the same forwards and backwards (a palindrome). Return its length. A subsequence does not need to be contiguous.

Take a moment to think about this before revealing the answer

Explain your reasoning or try an implementation before comparing answers.

Practice on LeetCode
CodingMedium

Palindromic Substrings

Question

Given a string s, return the number of palindromic substrings in it. Substrings of different lengths at different positions are counted separately, even if they consist of the same characters.

Take a moment to think about this before revealing the answer

Explain your reasoning or try an implementation before comparing answers.

Practice on LeetCode
CodingHard

Count Different Palindromic Subsequences

Question

Given a string s, return the number of different non-empty palindromic subsequences of s. Two subsequences are different if there is some position i at which they have a different character. The answer can be very large; return it modulo 10^9 + 7.

Take a moment to think about this before revealing the answer

Explain your reasoning or try an implementation before comparing answers.

Practice on LeetCode

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