add: SubstringAnagrams

Author: Jer3myYu

java/Sliding Windows/SubstringAnagrams.java

@@ -0,0 +1,42 @@
+public class SubstringAnagrams {
+    public int substringAnagrams(String s, String t) {
+        int lenS = s.length();
+        int lenT = t.length();
+        if (lenT > lenS) return 0;
+        int count = 0;
+        int[] expectedFreqs = new int[26];
+        int[] windowFreqs = new int[26];
+        // Populate 'expected_freqs' with the characters in string 't'.
+        for (char c : t.toCharArray()) {
+            expectedFreqs[c - 'a'] += 1;
+        }
+        int left, right;
+        left = right = 0;
+        while (right < lenS) {
+            // Add the character at the right pointer to 'window_freqs' 
+            // before sliding the window.
+            windowFreqs[s.charAt(right) - 'a'] += 1;
+            // If the window has reached the expected fixed length, we 
+            // advance the left pointer as well as the right pointer to 
+            // slide the window.
+            if (right - left + 1 == lenT) {
+                if (compareFreqs(windowFreqs, expectedFreqs)) {
+                    count += 1;
+                }
+                // Remove the character at the left pointer from 
+                // 'window_freqs' before advancing the left pointer.
+                windowFreqs[s.charAt(left) - 'a'] -= 1;
+                left += 1;
+            }
+            right += 1;
+        }
+        return count;
+    }
+
+    private boolean compareFreqs(int[] windowFreqs, int[] expectedFreqs) {
+        for (int i = 0; i < 26; i++) {
+            if (windowFreqs[i] != expectedFreqs[i]) return false;
+        }
+        return true;
+    }
+}