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Java-based LeetCode algorithm problem solutions, regularly updated
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package g2001_2100.s2055_plates_between_candles;

// #Medium #Array #String #Binary_Search #Prefix_Sum
// #2022_05_26_Time_10_ms_(92.49%)_Space_164.8_MB_(5.53%)

/**
 * 2055 - Plates Between Candles\.
 *
 * Medium
 *
 * There is a long table with a line of plates and candles arranged on top of it. You are given a **0-indexed** string `s` consisting of characters `'*'` and `'|'` only, where a `'*'` represents a **plate** and a `'|'` represents a **candle**.
 *
 * You are also given a **0-indexed** 2D integer array `queries` where queries[i] = [left_i, right_i] denotes the **substring** s[left_i...right_i] ( **inclusive** ). For each query, you need to find the **number** of plates **between candles** that are **in the substring**. A plate is considered **between candles** if there is at least one candle to its left **and** at least one candle to its right **in the substring**.
 *
 * *   For example, `s = "||**||**|*"`, and a query `[3, 8]` denotes the substring `"*||******|"`. The number of plates between candles in this substring is `2`, as each of the two plates has at least one candle **in the substring** to its left **and** right.
 *
 * Return _an integer array_ `answer` _where_ `answer[i]` _is the answer to the_ i^th _query_.
 *
 * **Example 1:**
 *
 * ![ex-1](https://assets.leetcode.com/uploads/2021/10/04/ex-1.png)
 *
 * **Input:** s = "\*\*|\*\*|\*\*\*|", queries = \[\[2,5],[5,9]]
 *
 * **Output:** [2,3]
 *
 * **Explanation:** - queries[0] has two plates between candles. - queries[1] has three plates between candles. 
 *
 * **Example 2:**
 *
 * ![ex-2](https://assets.leetcode.com/uploads/2021/10/04/ex-2.png)
 *
 * **Input:** s = "\*\*\*|\*\*|\*\*\*\*\*|\*\*||\*\*|\*", queries = \[\[1,17],[4,5],[14,17],[5,11],[15,16]]
 *
 * **Output:** [9,0,0,0,0]
 *
 * **Explanation:** - queries[0] has nine plates between candles. - The other queries have zero plates between candles. 
 *
 * **Constraints:**
 *
 * *   3 <= s.length <= 10⁵
 * *   `s` consists of `'*'` and `'|'` characters.
 * *   1 <= queries.length <= 10⁵
 * *   `queries[i].length == 2`
 * *   0 <= left_i <= right_i < s.length
**/
public class Solution {
    public int[] platesBetweenCandles(String s, int[][] queries) {
        char[] sa = s.toCharArray();
        int n = sa.length;
        int[] nextCandle = new int[n + 1];
        nextCandle[n] = -1;
        for (int i = n - 1; i >= 0; i--) {
            nextCandle[i] = sa[i] == '|' ? i : nextCandle[i + 1];
        }
        int[] prevCandle = new int[n];
        int[] prevPlates = new int[n];
        int countPlate = 0;
        if (sa[0] == '*') {
            countPlate = 1;
            prevCandle[0] = -1;
        }
        for (int i = 1; i < n; i++) {
            if (sa[i] == '|') {
                prevCandle[i] = i;
                prevPlates[i] = countPlate;
            } else {
                prevCandle[i] = prevCandle[i - 1];
                countPlate++;
            }
        }
        int len = queries.length;
        int[] res = new int[len];
        for (int i = 0; i < len; i++) {
            int[] query = queries[i];
            int start = query[0];
            int end = query[1];
            int curPlates = 0;
            int endCandle = prevCandle[end];
            if (endCandle >= start) {
                int startCandle = nextCandle[start];

                curPlates = prevPlates[endCandle] - prevPlates[startCandle];
            }
            res[i] = curPlates;
        }
        return res;
    }
}