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.

 

public class Solution {
    public int LengthOfLongestSubstringKDistinct(string s, int k) {
        if (s == null || k == 0) return 0;
        if (s.Length <= k) return s.Length;
        
        var dict = new Dictionary<char, int>();
        int start = 0, max = 0;
        for (int i = 0; i < s.Length; i++)
        {
            dict[s[i]] = i;
            
            if (dict.Count <= k)
            {
                max = Math.Max(max, i - start + 1);
            }
            else
            {
                int lastIndex = Int32.MaxValue;
                
                for (int j = start; j < i; j++)
                {
                    if (dict[s[j]] < lastIndex)
                    {
                        lastIndex = dict[s[j]]; 
                    }
                }
                
                dict.Remove(s[lastIndex]);
                start = lastIndex + 1;
            }
        }
        
        return max;
    }
}