longest substring with at least k repeated characters

Author: chen0040

.idea/workspace.xml

@@ -415,3 +415,7 @@ codes may not be optimized
 
 1. [LRU Cache](https://leetcode.com/problems/lru-cache/description/)
 1. [LFU Cache](https://leetcode.com/problems/lfu-cache/description/)
+
+### Day 20 - Medium
+
+1. [Longest Substring with at least k Repeating Characters](https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/description/)

README.md

@@ -0,0 +1,70 @@
+package com.github.chen0040.leetcode.day20.medium;
+
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+
+/**
+ * Created by xschen on 15/8/2017.
+ *
+ * link: https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/description/
+ */
+public class LongestSubstringWithAtLeastKRepeatedCharacters {
+   public class Solution {
+      public int longestSubstring(String s, int k) {
+         Map<Character, Integer> counts = new HashMap<Character, Integer>();
+         for(int i=0; i < s.length(); ++i) {
+            char c = s.charAt(i);
+            counts.put(c, counts.getOrDefault(c, 0) + 1);
+         }
+         StringBuilder sb = new StringBuilder();
+         for(Map.Entry<Character, Integer> entry : counts.entrySet()) {
+            if(entry.getValue().intValue() < k) {
+               sb.append(entry.getKey());
+            }
+         }
+         String lowFreqWords = sb.toString();
+         //System.out.println(lowFreqWords);
+
+         List<int[]> ranges = new ArrayList<int[]>();
+         int lo = 0;
+         int hi = 0;
+         if(!lowFreqWords.isEmpty()) {
+            for(int i=0; i < s.length(); ++i) {
+               char c = s.charAt(i);
+               if(lowFreqWords.indexOf(c) != -1) {
+                  hi = i - 1;
+                  if(hi >= lo) {
+                     ranges.add(new int[] { lo, hi});
+                     hi = -1;
+                  }
+                  lo = i + 1;
+               } else {
+                  hi++;
+               }
+            }
+            if(hi != -1) {
+               if(s.length()-1 >= lo){
+                  ranges.add(new int[] { lo, s.length() - 1});
+               }
+            }
+
+            int maxLen = 0;
+            for(int[] range : ranges){
+               //System.out.println("[" + range[0] + ", " + range[1] + "]");
+               String s2 = s.substring(range[0], range[1] + 1);
+               if(s2.length() < k) continue;
+
+               maxLen = Math.max(maxLen, longestSubstring(s2, k));
+            }
+            return maxLen;
+
+         } else {
+            return s.length();
+         }
+      }
+   }
+}