How to get the maximum count of repeated letters in a string? JavaScript

We have a string that contains some repeated letters like this:

const a = "fdsfjngjkdsfhhhhhhhhhhhfsdfsd";

Our job is to write a function that returns the count of maximum consecutive same letters in a streak. Like in the above string the letter 'h' appears for 11 times in a row consecutively, so our function should return 11 for this string.

This problem is a good candidate for the sliding window algorithm, where a stable window contains consecutive identical letters and one that contains different elements is unstable. The window adjusts by moving the start pointer when different characters are encountered.

Using Sliding Window Algorithm

The sliding window approach uses two pointers to track the current sequence of identical characters:

const a = "fdsfjngjkdsfhhhhhhhhhhhfsdfsd";
const findMaximumRepeating = str => {
    let max = 0;
    for(let start = 0, end = 1; end < str.length; ){
        if(str[end] === str[start]){
            if(max < end - start + 1){
                max = end - start + 1;
            }
            end++;
        } else {
            start = end;
        }
    }
    return max;
};
console.log(findMaximumRepeating(a));

Output

11

Alternative Method: Single Loop

Here's another approach using a single loop to count consecutive characters:

const findMaxRepeating = str => {
    if (!str) return 0;
    
    let maxCount = 1;
    let currentCount = 1;
    
    for (let i = 1; i < str.length; i++) {
        if (str[i] === str[i - 1]) {
            currentCount++;
            maxCount = Math.max(maxCount, currentCount);
        } else {
            currentCount = 1;
        }
    }
    return maxCount;
};

console.log(findMaxRepeating("fdsfjngjkdsfhhhhhhhhhhhfsdfsd"));
console.log(findMaxRepeating("aaabbbbcccc"));
console.log(findMaxRepeating("abcdef"));

11
4
1

How It Works

The sliding window algorithm works by:

• Maintaining two pointers: start and end
• Expanding the window when characters match
• Moving the start pointer to the current end position when characters differ
• Tracking the maximum window size encountered

Conclusion

Both methods efficiently find the maximum consecutive character count with O(n) time complexity. The sliding window approach is more versatile for similar problems, while the single loop method is simpler to understand.