solution: longest-repeating-character-replacement

Author: Jeongwon Na

longest-repeating-character-replacement/njngwn.java

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+// Time Complexity: O(n), n: s.length()
+// Space Complexity: O(1)
+class Solution {
+    public int characterReplacement(String s, int k) {
+        int[] charFreqArr = new int[26]; // s consists of only uppercase English letters
+        int maxFreq = 0;
+        int maxLength = 0;
+
+        for (int left = 0, right = 0; right < s.length(); right++) {
+            char letter = s.charAt(right);
+            int letterIdx = letter-'A';
+
+            charFreqArr[letterIdx]++;
+            maxFreq = Math.max(maxFreq, charFreqArr[letterIdx]);
+
+            // when left idx moves rightward? -> k + maxFreq < size of sliding window
+            // here, we don't neet to recalculate maxFreq because the point is to calculate 'best' maxFreq
+            if (maxFreq + k < right-left+1) {
+                char leftChar = s.charAt(left);
+                charFreqArr[leftChar-'A']--;
+                left++;
+            }
+
+            maxLength = Math.max(maxLength, right-left+1);
+        }
+
+        return maxLength;
+    }
+}
+
+// AABABBA, k=1, maxFreq=1, maxLength=1
+// l
+// r
+
+// AABABBA, k=1, maxFreq=2, maxLength=2
+// lr
+
+// AABABBA, k=1, maxFreq=2, maxLength=3
+// l r
+
+// AABABBA, k=1, maxFreq=3, maxLength=4
+// l  r
+
+// AABABBA, k=1, maxFreq=3, maxLength=4 ==> fail
+// l   r
+
+// AABABBA, k=1, maxFreq=3
+//  l   r