JavaAlgorithms/DynamicProgramming/LongestValidParentheses.java

package DynamicProgramming;

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;
      }
      
      System.out.println("Len is: " + getLongestValidParentheses(str));
    }

    sc.close();
  }
}