Add Longest Valid Parentheses algorithm(DP)

Dynamic Programming/LongestValidParentheses.java

@@ -0,0 +1,63 @@
+
+import java.util.Scanner;
+
+/**
+ * Given a string containing just the characters '(' and ')', find the length of
+ * the longest valid (well-formed) parentheses substring.
+ *
+ *
+ * @author Libin Yang (https://github.com/yanglbme)
+ * @since 2018/10/5
+ */
+
+public class LongestValidParentheses {
+
+	public static int getLongestValidParentheses(String s) {
+		if (s == null || s.length() < 2) {
+			return 0;
+		}
+		char[] chars = s.toCharArray();
+		int n = chars.length;
+		int[] res = new int[n];
+		res[0] = 0;
+		res[1] = chars[1] == ')' && chars[0] == '(' ? 2 : 0;
+
+		int max = res[1];
+
+		for (int i = 2; i < n; ++i) {
+			if (chars[i] == ')') {
+				if (chars[i - 1] == '(') {
+					res[i] = res[i - 2] + 2;
+				} else {
+					int index = i - res[i - 1] - 1;
+					if (index >= 0 && chars[index] == '(') {
+						// ()(())
+						res[i] = res[i - 1] + 2 + (index - 1 >= 0 ? res[index - 1] : 0);
+					}
+				}
+			}
+			max = Math.max(max, res[i]);
+		}
+
+		return max;
+
+	}
+
+	public static void main(String[] args) {
+		Scanner sc = new Scanner(System.in);
+
+		while (true) {
+			String str = sc.nextLine();
+			if ("quit".equals(str)) {
+				break;
+			}
+			int len = getLongestValidParentheses(str);
+			System.out.println(len);
+
+		}
+
+		sc.close();
+
+	}
+
+}