Substrings of Size Three with Distinct…

1. Define the problem

Substrings of Size Three with Distinct Characters

A string is good if there are no repeated characters. Given a string s, return the number of good substrings of length three in s. Note that if there are multiple occurrences of the same substring, every occurrence should be counted. A substring is a contiguous sequence of characters in a string. Slide a fixed length window of length 3 across the string and check whether all three characters inside are different.

Constraints

1 ≤ s.length ≤ 100
s consists of lowercase English letters.

Example

Inputs = "xyzzaz"

Output1

Explanation "xyz" is the only good substring of length three — the rest all repeat a character.

2. Visualize the solution

Check each window of size 3 for distinct characters

Statusgood

Window "xyz": x, y, z are all different — good substring. count = 1.

What happens in this step

window = "xyz"  [0, 2]

s[0]=x, s[1]=y, s[2]=z — all three different

Good substring — count = 1.

Step 1 of 4

Steps to visualize

Place a window of length 3 on the first three characters.
Check whether all three characters inside are different.
If they are, count it as a good substring.
Slide the window one step right and check again.
Continue until the window reaches the end of the string.

3. Walk through the code

Walk through the code

Same walkthrough, now with the code. Press Next to move one step and watch which lines run.

Check each window of size 3 for distinct characters

Statusgood

Window "xyz": x, y, z are all different — good substring. count = 1.

What happens in this step

window = "xyz"  [0, 2]

s[0]=x, s[1]=y, s[2]=z — all three different

Good substring — count = 1.

Step 1 of 4

4. Solution

Solution

solution.tsTypeScript

function countGoodSubstrings(s) {
  if (s.length < 3) {
    return 0;
  }

  let count = 0;
  for (let left = 0; left + 2 < s.length; left++) {
    const a = s[left];
    const b = s[left + 1];
    const c = s[left + 2];
    if (a !== b && b !== c && a !== c) {
      count++;
    }
  }

  return count;
}

Time: O(n)
Space: O(1)

5. Test cases

Test cases

Input	Expected	Covers
`s = "xyzzaz"`	`1`	example from the docstring
`s = "ab"`	`0`	string shorter than 3: no substrings of length 3 exist
`s = "abc"`	`1`	smallest valid input: exactly one good substring
`s = "aaa"`	`0`	smallest valid input: no distinct characters
`s = "abcabc"`	`4`	every substring of length 3 qualifies
`s = "aabbcc"`	`0`	no substrings qualify
`s = "aababcabc"`	`4`	larger, hand-verified case

Topic	Description
Sliding Window	Solve fixed-window problems: maximum sum of k elements, moving averages, and O(n) updates as the window slides.
Maximum Average Subarray I	Find the contiguous subarray of a fixed length with the largest average.
Longest Substring Without Repeating Characters	Find the length of the longest run of characters in a string with no repeats.
Permutation in String	Check whether one string contains a rearrangement of another string as a substring.
Minimum Size Subarray Sum	Find the shortest contiguous subarray whose sum is at least a target value.
Longest Repeating Character Replacement	Find the longest substring you can turn into one repeated letter with a limited number of swaps.