1358. Number of Substrings Containing All Three Characters
Description
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.
Solutions
Solution 1: Single Pass
We use an array $d$ of length $3$ to record the most recent occurrence of the three characters, initially all set to $-1$.
We traverse the string $s$. For the current position $i$, we first update $d[s[i]]=i$, then the number of valid strings is $\min(d[0], d[1], d[2]) + 1$, which is accumulated to the answer.
The time complexity is $O(n)$, where $n$ is the length of the string $s$. The space complexity is $O(1)$.
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