LongestValidParentheses32.java

/**
 * Given a string containing just the characters '(' and ')', find the length
 * of the longest valid (well-formed) parentheses substring.
 * 
 * Example 1:
 * Input: "(()"
 * Output: 2
 * Explanation: The longest valid parentheses substring is "()"
 * 
 * Example 2:
 * Input: ")()())"
 * Output: 4
 * Explanation: The longest valid parentheses substring is "()()"
 */


public class LongestValidParentheses32 {
    public int longestValidParentheses(String s) {
        int len = s.length();
        if (len <= 1) return 0;
        boolean[] valid = new boolean[len];
        Stack<Integer> stack = new Stack<>();
        char[] chars = s.toCharArray();
        for (int i=0; i<len; i++) {
            if (chars[i] == '(') {
                stack.push(i);
            } else {
                if (!stack.isEmpty())  {
                    int left = stack.pop();
                    valid[left] = true;
                    valid[i] = true;
                }
            }
        }
        
        int curr = 0;
        int max = 0;
        for (int i=0; i<len; i++) {
            if (valid[i]) {
                curr++;
            } else {
                max = Math.max(max, curr);
                curr = 0;
            }
        }
        return Math.max(max, curr);
    }


    /**
     * https://leetcode.com/problems/longest-valid-parentheses/solution/
     */
    public int longestValidParentheses2(String s) {
        int maxans = 0;
        Stack<Integer> stack = new Stack<>();
        stack.push(-1);
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                stack.push(i);
            } else {
                stack.pop();
                if (stack.empty()) {
                    stack.push(i);
                } else {
                    maxans = Math.max(maxans, i - stack.peek());
                }
            }
        }
        return maxans;
    }


    /**
     * https://leetcode.com/problems/longest-valid-parentheses/solution/
     */
    public int longestValidParentheses3(String s) {
        int left = 0, right = 0, maxlength = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                left++;
            } else {
                right++;
            }
            if (left == right) {
                maxlength = Math.max(maxlength, 2 * right);
            } else if (right >= left) {
                left = right = 0;
            }
        }
        left = right = 0;
        for (int i = s.length() - 1; i >= 0; i--) {
            if (s.charAt(i) == '(') {
                left++;
            } else {
                right++;
            }
            if (left == right) {
                maxlength = Math.max(maxlength, 2 * left);
            } else if (left >= right) {
                left = right = 0;
            }
        }
        return maxlength;
    }

}