LeetCode Plates Between Candles Solution
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
'|' only, where a
'*' represents a plate and a
'|' represents a candle.
You are also given a 0-indexed 2D integer array
queries[i] = [lefti, righti] denotes the substring
s[lefti...righti] (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
7), as each of the two plates has at least one candle in the substring to its left and right.
Return an integer array
answer[i] is the answer to the
Input: s = "**|**|***|", queries = [[2,5],[5,9]]
- queries has two plates between candles.
- queries has three plates between candles.
Input: s = "***|**|*****|**||**|*", queries = [[1,17],[4,5],[14,17],[5,11],[15,16]]
- queries has nine plates between candles.
- The other queries have zero plates between candles.
3 <= s.length <= 105
1 <= queries.length <= 105
queries[i].length == 2
0 <= lefti <= righti < s.length
Problem Link: https://leetcode.com/problems/plates-between-candles/
To find the number of plates for each query
(qleft, qright), we set up an array
candles to store the candles' indices, so that we could later do basic arithmetic on the indices to find the number of plates.
First, we need to find the outside candles' indices in the input
s, this can be done via binary search in
candles. We will find
right_pos indicating the outside candle's position in
Then, We know that the number of plates is given by the interval between the two bounding candles subtracted by the number of candles in between.
With the indices
right_pos, we can derive the number of plates to be
(candles[right_pos] - candles[left_pos]) - (right_pos - left_pos).
1def platesBetweenCandles(s, queries): 2 candles =  3 for i in range(len(s)): 4 if s[i] == '|': candles.append(i) 5 6 res =  7 for qleft, qright in queries: 8 left_pos, right_pos = -1, -1 9 10 # 1. find index of first candle that comes after qleft 11 left, right = 0, len(candles)-1 12 while left <= right: 13 mid = (left+right) // 2 14 if candles[mid] >= qleft: 15 right = mid - 1 16 left_pos = mid 17 else: 18 left = mid + 1 19 20 # 2. find index of last candle that comes before qright 21 left, right = 0, len(candles)-1 22 while left <= right: 23 mid = (left+right) // 2 24 if candles[mid] <= qright: 25 left = mid + 1 26 right_pos = mid 27 else: 28 right = mid - 1 29 30 # result = range between two outermost candles - candle count in between 31 if (left_pos != -1) & (right_pos!= -1) & (right_pos > left_pos): 32 res.append((candles[right_pos] - candles[left_pos]) - (right_pos - left_pos)) 33 else: 34 res.append(0) 35 return res