If you know how to find LCS between 2 strings, then this problems can be reduced to finding the LCS between the original string and its reversed form:

```
public int longestPalindromeSubseq(String s) {
if (s == null || s.isEmpty()) return 0;
int len = s.length();
int[][] dp = new int[len + 1][len + 1];
String t = new StringBuilder(s).reverse().toString();
for (int i = len - 1; i >= 0; i--) {
for (int j = len - 1; j >= 0; j--) {
if (s.charAt(i) == t.charAt(j)) {
dp[i][j] = 1 + dp[i+1][j+1];
} else {
dp[i][j] = Math.max(dp[i+1][j], dp[i][j+1]);
}
}
}
return dp[0][0];
}
```