LeetCode #424: Longest Repeating Character Replacement (C/C++).

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source: https://leetcode.com/problems/longest-repeating-character-replacement
C/C++ Solution to LeetCode problem 424. Longest Repeating Character Replacement.

Problem

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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.

Examples

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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

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

Solution

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We start with a window size = 1, and we start growing the window.

• Using a hash table.
• We increase the count for the added character.
• We track max frequency (most repeated character)
• if the window size - the max freq is > than the allowed elements to change
  • We decrease the count of the character.
  • We increase the initial index for our final string.
• We update the new max size of the final string.

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class Solution {
public:
  int characterReplacement(string s, int k) {
    int j = 0;
    int maxfq = 0;
    int result = 0;
    unordered_map<char, int> mp;
    
    for (int i=0; i<s.size(); i+=1) {
      mp[s[i]] += 1;
      maxfq = max(maxfq, mp[s[i]]);
      if (i - j + 1 - maxfq > k) {
        mp[s[j]] -= 1;
        j += 1;
      }
      result = max(res, i-j+1);
    }

    return result;
  }
};