g1901_2000.s1930_unique_length_3_palindromic_subsequences.Solution Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of leetcode-in-java21 Show documentation

Java-based LeetCode algorithm problem solutions, regularly updated

There is a newer version: 1.38

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;

/**
 * 1930 - Unique Length-3 Palindromic Subsequences\.
 *
 * Medium
 *
 * Given a string `s`, return _the number of **unique palindromes of length three** that are a **subsequence** of_ `s`.
 *
 * Note that even if there are multiple ways to obtain the same subsequence, it is still only counted **once**.
 *
 * A **palindrome** is a string that reads the same forwards and backwards.
 *
 * A **subsequence** of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.
 *
 * *   For example, `"ace"` is a subsequence of `"abcde"`.
 *
 * **Example 1:**
 *
 * **Input:** s = "aabca"
 *
 * **Output:** 3
 *
 * **Explanation:** The 3 palindromic subsequences of length 3 are: 
 *
 * - "aba" (subsequence of "aabca") 
 *
 * - "aaa" (subsequence of "aabca") 
 *
 * - "aca" (subsequence of "aabca")
 *
 * **Example 2:**
 *
 * **Input:** s = "adc"
 *
 * **Output:** 0
 *
 * **Explanation:** There are no palindromic subsequences of length 3 in "adc".
 *
 * **Example 3:**
 *
 * **Input:** s = "bbcbaba"
 *
 * **Output:** 4
 *
 * **Explanation:** The 4 palindromic subsequences of length 3 are: 
 *
 * - "bbb" (subsequence of "bbcbaba") 
 *
 * - "bcb" (subsequence of "bbcbaba") 
 *
 * - "bab" (subsequence of "bbcbaba") 
 *
 * - "aba" (subsequence of "bbcbaba")
 *
 * **Constraints:**
 *
 * *   3 <= s.length <= 10⁵
 * *   `s` consists of only lowercase English letters.
**/
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;
    }
}