Add optimized Java solution for Valid Parenthesis String (LeetCode 678)

Java/ValidParenthesisString_Enhanced.java

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+/**
+ * ✅ Valid Parenthesis String — LeetCode 678
+ *
+ * Problem
+ * -------
+ * Given a string s containing only the characters '(', ')' and '*', where '*'
+ * can be treated as '(' or ')' or an empty string, determine if s is a valid
+ * parenthesis string.
+ *
+ * A string is valid if:
+ * - Every '(' has a corresponding ')' that comes after it.
+ * - Every ')' has a corresponding '(' that comes before it.
+ * - Parentheses are properly nested.
+ *
+ * Key Idea (Greedy Lower/Upper Bound)
+ * -----------------------------------
+ * Track a RANGE of how many '(' can be "open" at each position:
+ * - lo = the minimum possible number of open '(' so far if we use '*' as ')' when helpful
+ * - hi = the maximum possible number of open '(' so far if we use '*' as '(' when helpful
+ *
+ * For each character:
+ * - '('  => lo++, hi++
+ * - ')'  => lo = max(0, lo - 1), hi--
+ * - '*'  => lo = max(0, lo - 1)   // treat '*' as ')'
+ *           hi++                  // OR treat '*' as '('
+ *
+ * If at any time hi < 0, we have more ')' than we can match => invalid.
+ * At the end, we must be able to close all opens in the "best" way: lo == 0.
+ *
+ * Time/Space:
+ * - Time: O(n)
+ * - Space: O(1)
+ *
+ * Alternative Approaches (for reference)
+ * --------------------------------------
+ * 1) Two-pass counting:
+ *    - Left-to-right: treat '*' as '(' and ensure no premature neg balance.
+ *    - Right-to-left: treat '*' as ')' and ensure no premature neg balance.
+ *    If both passes succeed, the string is valid. O(n) time, O(1) space.
+ *
+ * 2) Stack-based:
+ *    - One stack for '(' indices, one for '*' indices. Match ')' greedily,
+ *      then match remaining '(' with '*' coming after them. O(n) time, O(n) space.
+ *
+ * References
+ * ----------
+ * - Problem: https://leetcode.com/problems/valid-parenthesis-string/
+ * - Editorial: https://leetcode.com/problems/valid-parenthesis-string/solutions/
+ *
+ * This file includes:
+ * - Greedy O(n)/O(1) solution: checkValidString
+ * - Two-pass O(n)/O(1) solution: checkValidStringTwoPass
+ * - Sample main() with tests
+ */
+public class ValidParenthesisString {
+
+    /**
+     * Greedy lower/upper bound solution.
+     *
+     * @param s input string consisting of '(', ')', '*'
+     * @return true if s can be a valid parenthesis string, false otherwise
+     */
+    public static boolean checkValidString(String s) {
+        int lo = 0; // min possible open '('
+        int hi = 0; // max possible open '('
+
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            if (c == '(') {
+                lo++;
+                hi++;
+            } else if (c == ')') {
+                if (lo > 0) lo--;
+                hi--;
+            } else { // '*'
+                if (lo > 0) lo--;
+                hi++;
+            }
+            if (hi < 0) return false; // too many ')'
+        }
+
+        return lo == 0; // all opens can be closed
+    }
+
+    /**
+     * Two-pass linear-time solution.
+     * Pass 1 (L->R): treat '*' as '(' to ensure we never run out of opens.
+     * Pass 2 (R->L): treat '*' as ')' to ensure we never run out of closes.
+     */
+    public static boolean checkValidStringTwoPass(String s) {
+        int balance = 0;
+
+        // Left-to-right: treat '*' as '('
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            balance += (c == ')') ? -1 : 1;
+            if (balance < 0) return false;
+        }
+
+        // Right-to-left: treat '*' as ')'
+        balance = 0;
+        for (int i = s.length() - 1; i >= 0; i--) {
+            char c = s.charAt(i);
+            balance += (c == '(') ? -1 : 1;
+            if (balance < 0) return false;
+        }
+
+        return true;
+    }
+
+    // Helper to print results neatly
+    private static void runAndPrint(String s) {
+        boolean greedy = checkValidString(s);
+        boolean twoPass = checkValidStringTwoPass(s);
+        System.out.printf("s=\"%s\" -> greedy=%s, twoPass=%s%n", s, greedy, twoPass);
+    }
+
+    public static void main(String[] args) {
+        // Basic sanity tests (including LeetCode examples)
+        runAndPrint("");
+        runAndPrint("()");
+        runAndPrint("(*)");
+        runAndPrint("(*))");
+        runAndPrint("(*()");
+        runAndPrint(")*(");
+        runAndPrint("(()");
+        runAndPrint("())");
+        runAndPrint("()*");
+        runAndPrint("((*)");
+        runAndPrint("(*)))");
+        runAndPrint("((()))");
+        runAndPrint("((*)*)");
+        runAndPrint("(((******))");
+        runAndPrint("((**))");
+        runAndPrint("((((");
+        runAndPrint("))))");
+        runAndPrint("*");
+        runAndPrint("(*");
+        runAndPrint(")*");
+        runAndPrint("((*)*)*");
+    }
+}