LeetCode-in-Java
730. Count Different Palindromic Subsequences
Hard
Given a string s, return the number of different non-empty palindromic subsequences in s. Since the answer may be very large, return it modulo 109 + 7.
A subsequence of a string is obtained by deleting zero or more characters from the string.
A sequence is palindromic if it is equal to the sequence reversed.
Two sequences a1, a2, ... and b1, b2, ... are different if there is some i for which ai != bi.
Example 1:
Input: s = “bccb”
Output: 6
Explanation: The 6 different non-empty palindromic subsequences are ‘b’, ‘c’, ‘bb’, ‘cc’, ‘bcb’, ‘bccb’. Note that ‘bcb’ is counted only once, even though it occurs twice.
Example 2:
Input: s = “abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba”
Output: 104860361
Explanation: There are 3104860382 different non-empty palindromic subsequences, which is 104860361 modulo 109 + 7.
Constraints:
1 <= s.length <= 1000s[i]is either'a','b','c', or'd'.
Solution
public class Solution {
public int countPalindromicSubsequences(String s) {
int big = 1000000007;
int len = s.length();
if (len < 2) {
return len;
}
int[][] dp = new int[len][len];
for (int i = 0; i < len; i++) {
char c = s.charAt(i);
int deta = 1;
dp[i][i] = 1;
int l2 = -1;
for (int j = i - 1; j >= 0; j--) {
if (s.charAt(j) == c) {
if (l2 < 0) {
l2 = j;
deta = dp[j + 1][i - 1] + 1;
} else {
deta = dp[j + 1][i - 1] - dp[j + 1][l2 - 1];
}
deta = deal(deta, big);
}
dp[j][i] = deal(dp[j][i - 1] + deta, big);
}
}
return deal(dp[0][len - 1], big);
}
private int deal(int x, int big) {
x %= big;
if (x < 0) {
x += big;
}
return x;
}
}