Substrings of Size Three with Distinct Characters

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A string is good if there are no repeated characters. Given string s, return the number of good substrings of length exactly 3.

Example 1
Input: "xyzzaz" → Output: 1 (only "xyz" is good)
Example 2
Input: "aababcabc" → Output: 4

Solution Approach

Slide a window of fixed size 3, check if all 3 characters are distinct.

  • Iterate i from 0 to n-3.
  • Extract s[i], s[i+1], s[i+2].
  • If all three are distinct (s[i]!=s[i+1] && s[i+1]!=s[i+2] && s[i]!=s[i+2]): count++.
Java
class Solution {
    public int countGoodSubstrings(String s) {
        int count = 0;
        for (int i = 0; i + 2 < s.length(); i++) {
            char a = s.charAt(i), b = s.charAt(i + 1), c = s.charAt(i + 2);
            if (a != b && b != c && a != c) count++;
        }
        return count;
    }
}
  • Time Complexity: O(N)
  • Space Complexity: O(1)