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What if we divide the string into substrings containing only one distinct character with maximal lengths?
Now that you have sub-strings with only one distinct character, Try to come up with a formula that counts the number of its sub-strings.
Alternatively, Observe that the constraints are small so you can use brute force.
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A sequence of k identical characters can form substrings of length 1 through k. The number of such substrings follows the arithmetic series 1 + 2 + ... + k, which simplifies to k*(k+1)/2.
Yes, similar substring counting problems appear in coding interviews at companies like Amazon and Google. They test your understanding of string traversal, pattern observation, and converting brute-force ideas into linear-time solutions.
No special data structure is required. A simple variable to track the current run length of identical characters is enough, making the solution space-efficient.
The optimal approach scans the string and counts lengths of consecutive identical characters. For each block of length k, the number of valid substrings is k*(k+1)/2. Summing this for all blocks gives the total in O(n) time.