Given a string s, find the longest palindromic subsequence's length in s. You may assume that the maximum length of s is 1000.
Example 1:
Input:
"bbbab"
Output:
4
One possible longest palindromic subsequence is "bbbb".
Example 2:
Input:
"cbbd"
Output:
2
One possible longest palindromic subsequence is "bb".
Constraints:
1 <= s.length <= 1000
s consists only of lowercase English letters.
class Solution {
public int longestPalindromeSubseq(String s) {
// dp[i][j] -> length of longest palindromic subsequence between index i and j
int dp[][] = new int[s.length()][s.length()];
// now the base case will be that when i=j then longest sequence will be that
// character itself
// Therefore ans=1
for (int i = 0; i < s.length(); i++)
dp[i][i] = 1;
// Remember pointer j will always be ahead of pointer i
// Look at dp[i][j] calls like putting 2 pointers at string
// Now to use our base cases we need this flow of loop
for (int i = s.length() - 1; i >= 0; i--) {
for (int j = i + 1; j < s.length(); j++) {
if (s.charAt(i) == s.charAt(j)) {
dp[i][j] = dp[i + 1][j - 1] + 2;
} else {
dp[i][j] = Math.max(dp[i + 1][j], dp[i][j - 1]);
}
}
}
return dp[0][s.length() - 1];
}
}