Longest Repeating Character Replacement

Description

You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times.

Return the length of the longest substring containing the same letter you can get after performing the above operations.

 

Example 1:

Input: s = "ABAB", k = 2
Output: 4
Explanation: Replace the two 'A's with two 'B's or vice versa.

Example 2:

Input: s = "AABABBA", k = 1
Output: 4
Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA".
The substring "BBBB" has the longest repeating letters, which is 4.

 

Constraints:

• 1 <= s.length <= 105
• s consists of only uppercase English letters.
• 0 <= k <= s.length

Solution(javascript)

/**
 * @param {string} s
 * @param {number} k
 * @return {number}
 */
var characterReplacement = function(s, k) {
    // if empty string
    if (!s || s.length === 0) return 0;
    // Track letters
    let map = {};
    let max = 0;
    let currentCountMax = 0;
    let left = 0;
    let currentLetter;
    for (let i = 0; i < s.length; i++) {
        currentLetter = s[i];
        // Count the occurence of the current letter (in the present window)
        // Track num of char occurances
        map[currentLetter] ? map[currentLetter]++ : map[currentLetter] = 1;
        // Update cur_count_max (in the present window)
        if(map[currentLetter] > currentCountMax) currentCountMax = map[currentLetter];
        // Sliding window: Check if the window we are checking is valid or not
        // In case the size of window is bigger than cur_count_max plus max change(k)
        // decrease the count of the leftmost letter and then slide left (left++)
        if (i - left + 1 > currentCountMax + k) {
            map[s[left]] -= 1;
            left++
        } else {
            max = Math.max(max, i - left + 1)
        }
    }
    return max;
};