Create longest-substring-with-at-least-k-repeating-characters.cpp

Author: kamyu104

C++/longest-substring-with-at-least-k-repeating-characters.cpp

@@ -0,0 +1,40 @@
+// Time:  O(26 * n) = O(n)
+// Space: O(26) = O(1)
+
+// Recursive solution.
+class Solution {
+public:
+    int longestSubstring(string s, int k) {
+        return longestSubstringHelper(s, k, 0, s.size());
+    }
+
+private:
+    int longestSubstringHelper(const string& s, int k, int start, int end) {
+        vector<int> count(26);
+        for (int i = start; i < end; ++i) {
+            ++count[s[i] - 'a'];
+        }
+        
+        int max_len = 0;
+        for (int i = start; i < end;) {
+            while (i < end && count[s[i] - 'a'] < k) {
+                ++i;
+            }
+            if (i == end) {
+                break;
+            }
+
+            int j = i;
+            while (j < end && count[s[j] - 'a'] >= k) {
+                ++j;
+            }
+            if (i == start && j == end) {
+                return end - start; 
+            }
+
+            max_len = max(max_len, longestSubstringHelper(s, k, i, j));
+            i = j;
+        }
+        return max_len;
+    }
+};