g1901_2000.s1930_unique_length_3_palindromic_subsequences.Solution Maven / Gradle / Ivy

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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;
    }
}