Added MinimumWindowSubstring.java implementing sliding window approach

Author: jaimin45-art

src/main/java/com/thealgorithms/strings/MinimumWindowSubstring.java

@@ -0,0 +1,80 @@
+package com.thealgorithms.strings;
+
+import java.util.Arrays;
+
+/**
+ * Minimum Window Substring
+ *
+ * Given two strings s and t, return the minimum window substring of s such that
+ * every character in t (including duplicates) is included in the window.
+ *
+ * If there is no such substring, return an empty string "".
+ *
+ * Approach: sliding window + frequency table.
+ *
+ * Time complexity: O(n + m) where n = s.length(), m = t.length()
+ * Space complexity: O(1) if using fixed-size arrays for ASCII (256).
+ */
+public final class MinimumWindowSubstring {
+
+    private MinimumWindowSubstring() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    /**
+     * Returns the minimum window substring of s that contains all chars from t.
+     *
+     * @param s source string
+     * @param t target string
+     * @return shortest substring of s containing all chars of t, or "" if none
+     */
+    public static String minWindow(String s, String t) {
+        if (s == null || t == null || s.length() < t.length() || t.length() == 0) {
+            return "";
+        }
+
+        int[] need = new int[256];
+        for (char c : t.toCharArray()) {
+            need[c]++;
+        }
+
+        int required = 0;
+        for (int i = 0; i < 256; i++) {
+            if (need[i] > 0) required++;
+        }
+
+        int l = 0, r = 0;
+        int formed = 0;
+        int[] window = new int[256];
+        int minLen = Integer.MAX_VALUE;
+        int minLeft = 0;
+
+        while (r < s.length()) {
+            char c = s.charAt(r);
+            window[c]++;
+            if (need[c] > 0 && window[c] == need[c]) {
+                formed++;
+            }
+
+            // Try and contract the window till the point it ceases to be 'desirable'.
+            while (l <= r && formed == required) {
+                // Update result
+                if (r - l + 1 < minLen) {
+                    minLen = r - l + 1;
+                    minLeft = l;
+                }
+
+                char d = s.charAt(l);
+                window[d]--;
+                if (need[d] > 0 && window[d] < need[d]) {
+                    formed--;
+                }
+                l++;
+            }
+
+            r++;
+        }
+
+        return minLen == Integer.MAX_VALUE ? "" : s.substring(minLeft, minLeft + minLen);
+    }
+}