[njngwn] WEEK 08 solutions

longest-repeating-character-replacement/njngwn.java

@@ -0,0 +1,48 @@
+// 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

palindromic-substrings/njngwn.java

@@ -0,0 +1,36 @@
+// Time Complexity: O(n*n), n: s.length()
+// Space Complexity: O(1)
+class Solution {
+    public int countSubstrings(String s) {
+        int cnt = 0;
+
+        for (int i = 0; i < s.length(); i++, cnt++) {   // i: center of palindrom
+            // odd number palindrom: e.g. aba
+            int left = i-1, right = i+1;
+
+            while (left >= 0 && right < s.length()) {
+                if (s.charAt(left) != s.charAt(right)) {
+                    break;
+                }
+                cnt++;
+                left--;
+                right++;
+            }
+
+            // even number palindrom: e.g. abba
+            left = i;
+            right = i+1;
+
+            while (left >= 0 && right < s.length()) {
+                if (s.charAt(left) != s.charAt(right)) {
+                    break;
+                }
+                cnt++;
+                left--;
+                right++;
+            }
+        }
+
+        return cnt;
+    }
+}