feat : longest-repeating-character-replacement

Author: ekgns33

longest-repeating-character-replacement/ekgns33.java

@@ -0,0 +1,119 @@
+/**
+
+ input : String s
+ output : after k times operation, return length of longest substring
+ that consist of same letters
+
+ example
+ AAAA << 4
+ AAAB << AAA << 3
+
+ constraints:
+ 1) input string can be empty?
+ nope. at least one letter
+ 2) string s contains whitespace? non-alphabetical chars?
+ nope. only uppercase English letters
+ 3) range of k?
+ [0, n], when n is the length of string s
+
+ ABAB
+ AAAB
+ AAAA
+
+ AABABBA
+ AAAABBA
+ AABBBBA
+
+ AABABBA
+ l
+ r
+ count : 1
+ 4
+
+ solution 1: brute force
+ ds : array
+ algo : for loop
+
+ nested for loop
+ iterate through the string s from index i = 0 to n
+ set current character as 'target'
+ set count = 0
+ iterate through the string s from index j = i + 1 to n
+ if next character is identical to 'target'
+ continue;
+ else
+ increase count;
+
+ if count > k
+ break and save length
+
+ if count <=k compare length with max
+ ABAB
+ i
+ j
+ target = A
+ count = 1
+ ABAB << count = 2, length = 4
+ tc : O(n^2)
+ sc : O(1)
+
+ solution 2: two pointer
+
+ AABCABBA
+ l
+ r
+ A : 1
+ B : 1
+ C : 1
+ count = 1
+ res = 4
+ set two pointer l and r
+ set count to 0;
+ set maxFrequency character s[0];
+
+ move r pointer to right
+ if current char is not maxFreqChar
+ count + 1;
+ else
+ continue;
+
+ if count > k
+ while count <= k
+ move leftpointer right;
+ update frequency
+ set max Frequency character at each iteration O(26)
+ update count
+
+ tc: O(n)
+ sc : O(26) ~= O(1)
+ */
+
+class Solution {
+  public int characterReplacement(String s, int k) {
+    int l = 0, r = 1, maxFreq = 1, res = 1;
+    int n = s.length();
+    int[] freq = new int[26];
+    // base case;
+    char maxFreqChar = s.charAt(0);
+    freq[maxFreqChar - 'A']++;
+    // loop
+    while (r < n) {
+      char next = s.charAt(r);
+      maxFreq = Math.max(maxFreq, ++freq[next - 'A']);
+
+      while(r - l + 1 - maxFreq > k) {
+        freq[s.charAt(l) - 'A']--;
+        l++;
+        for(int i = 0; i <26; i++) {
+          if(freq[i] > maxFreq) {
+            maxFreq = freq[i];
+          }
+        }
+      }
+      res = Math.max(res, r - l+1);
+      r++;
+    }
+
+    return res;
+  }
+}