LeetCode 516 Longest Palindromic Subsequence - Implementation

Here's the solution:

function longestPalindromeSubseq(s)
    n := length of s
    dp := 2D (two-dimensional) array of size n × n, initialized to 0
    // Base case: single characters
    for i from 0 to n - 1
        dp[i][i] := 1
    // Fill by increasing length
    for len from 2 to n
        for i from 0 to n - len
            j := i + len - 1
            if s[i] = s[j] then
                dp[i][j] := dp[i + 1][j - 1] + 2
            else
                dp[i][j] := max(dp[i + 1][j], dp[i][j - 1])
    return dp[0][n - 1]

Time: $O(n^2)$. Space: $O(n^2)$. This is the standard interval DP pattern: iterate by length, compute shorter ranges first.

Time complexity: $O(n^2)$.

Space complexity: $O(n^2)$.