[LeetCode] 340. Longest Substring with At Most K Distinct Characters

Given a string, find the length of the longest substring T that contains at most k distinct characters.
For example, Given s = “eceba” and k = 2,
T is "ece" which its length is 3.

Thought process:
Sliding window. Use a hash map of character => integer to keep track of the count of each character in the current window.

Solution:

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class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        Map<Character, Integer> map = new HashMap<>();
        int i = 0;
        int longest = 0;
        
        for (int j = 0; j < s.length(); j++) {
            int count = map.getOrDefault(s.charAt(j), 0);
            map.put(s.charAt(j), count + 1);
            
            if (map.size() > k) {
                while (i < s.length() && map.size() > k) {
                    char c = s.charAt(i);
                    map.put(c, map.get(c) - 1);
                    if (map.get(c) == 0) {
                        map.remove(c);
                    }
                    i++;
                }
            }
            longest = Math.max(longest, j - i + 1);
        }
        return longest;
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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