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package g1901_2000.s1930_unique_length_3_palindromic_subsequences;
// #Medium #String #Hash_Table #Prefix_Sum #2022_05_15_Time_31_ms_(93.44%)_Space_50.4_MB_(66.39%)
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
public class Solution {
public int countPalindromicSubsequence(String s) {
int[] last = new int[26];
Arrays.fill(last, -1);
for (int i = s.length() - 1; i >= 0; i--) {
if (last[s.charAt(i) - 'a'] == -1) {
last[s.charAt(i) - 'a'] = i;
}
}
int ans = 0;
int[] count = new int[26];
Map first = new HashMap<>();
for (int i = 0; i < s.length(); i++) {
int cur = s.charAt(i) - 'a';
if (last[cur] - i <= 1 && !first.containsKey(cur)) {
last[cur] = -1;
}
if (last[cur] == i) {
int[] oldCount = first.get(cur);
for (int j = 0; j < 26; j++) {
if (count[j] - oldCount[j] > 0) {
ans++;
}
}
}
count[cur]++;
if (last[cur] > -1 && !first.containsKey(cur)) {
first.put(cur, count.clone());
}
}
return ans;
}
}