http://www.lintcode.com/en/problem/longest-substring-with-at-most-k-distinct-characters/

Given a string s, find the length of the longest substring T that contains at most k distinct characters.

Example For example, Given s = "eceba", k = 3, T is "eceb" which its length is 4.

public class Solution {
    /*
     * @param s: A string
     * @param k: An integer
     * @return: An integer
     */
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        // write your code here
        int maxLen = 0;

        // Key: letter; value: the number of occurrences.
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        int i = 0, j = 0;
        char c;
        for (i = 0; i < s.length(); i++) {
            while (j < s.length()) {
                c = s.charAt(j);
                if (map.containsKey(c)) {
                    map.put(c, map.get(c) + 1);
                } else {
                    if (map.size() == k) {
                        break;
                    }
                    map.put(c, 1);
                }
                j++;
            }

        maxLen = Math.max(maxLen, j - i);
        c = s.charAt(i);
        if(map.containsKey(c)){
          int count = map.get(c);
          if (count > 1) {
            map.put(c, count - 1);
          } else {
            map.remove(c);
          }
        }
      }
      return maxLen; 
  }  
}