LeetCode 1358. Number of Substrings Containing All Three Characters

原题链接在这里：https://leetcode.com/problems/number-of-substrings-containing-all-three-characters/description/

题目：

Given a string s consisting only of characters a, b and c.

Return the number of substrings containing at least one occurrence of all these characters a, b and c.

Example 1:

Input: s = "abcabc"
Output: 10
Explanation: The substrings containing at least one occurrence of the characters a, b and c are "abc", "abca", "abcab", "abcabc", "bca", "bcab", "bcabc", "cab", "cabc" and "abc" (again).

Example 2:

Input: s = "aaacb"
Output: 3
Explanation: The substrings containing at least one occurrence of the characters a, b and c are "aaacb", "aacb" and "acb".

Example 3:

Input: s = "abc"
Output: 1

Constraints:

3 <= s.length <= 5 x 10^4
s only consists of a, b or c characters.

题解：

Have walker and runner both move to the right. When all a, b, c count is larger than 0, move the walker.

res += walker is because there could be walker substrings fulfill the requirement.

e.g. aaabc, walker could move to 3, res += walker. Since there are 3 substrings aaabc, aabc and abc.

Time Complexity: O(n). n = s.length().

Space: O(1).

AC Java:

 1 class Solution {
 2     public int numberOfSubstrings(String s) {
 3         int [] map = new int[3];
 4         int walker = 0;
 5         int runner = 0;
 6         int res = 0;
 7         int n = s.length();
 8         while(runner < n){
 9             map[s.charAt(runner++) - 'a']++;
10             while(map[0] > 0 && map[1] > 0 && map[2] > 0){
11                 map[s.charAt(walker++) - 'a']--;
12             }
13 
14             res += walker;
15         }
16 
17         return res;
18     }
19 }