32. Longest Valid Parentheses

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 "()()"

class Solution {
    
    public int longestValidParentheses(String s) {
        int n = s.length();
        int []f = new int[n];
        int []stack = new int[n];
        int ans = 0;
        int k = 0;
        for (int i = 0; i < n; ++i) {
            if (s.charAt(i) == '(') {
                stack[k++] = i;
            }
            if (s.charAt(i) == ')') {
                if (k > 0) {
                    f[i] = i - stack[k - 1] + 1;
                    if (stack[k - 1] - 1 >= 0 && f[stack[k - 1] - 1] > 0)
                        f[i] += f[stack[k - 1] - 1];
                    k--;
                } else {
                    k = 0;
                    
                }
                
            }
        }
        for (int i = 0; i < n; ++i) {
            ans = Math.max(ans, f[i]);
        }
        
        return ans;
    }
}

public class Solution {

    public int longestValidParentheses(String s) {
        int n = s.length();
        int []dp = new int[n + 1];
        for (int i = 0; i < n; ++i) {
            if (s.charAt(i) == ')') {
                if (i - 1 >= 0 && s.charAt(i - 1) == '(') {
                    
                    dp[i + 1] = 2 + dp[i - 2 + 1];
                    
                }
                if ( i - 1 - dp[i] >= 0 && s.charAt(i - 1 - dp[i]) == '(') {
                    dp[i + 1] = Math.max(dp[i + 1], 2 + dp[i] + dp[i - 1 - dp[i]] );
                }
                
            }
        }
        int ans = 0;
        for (int i = 1; i <= n; ++i) ans = Math.max(ans, dp[i]);
        //for (int i = 1; i <= n; ++i) System.out.print(dp[i] + " ");
        return ans;
    }
}

public class Solution {

    public int longestValidParentheses(String s) {
        int n = s.length();
        int left = 0, right = 0;
        int ans = 0;
        for (int i = 0; i < n; ++i) {
            if (s.charAt(i) == '(') left++;
            if (s.charAt(i) == ')') right++;
            if (left == right) {
                ans = Math.max(ans, 2*left);
            } else if(left < right) {
                left = 0;
                right = 0;
                
            }
        }
        left = right = 0;
        for (int i = n - 1; i >= 0; --i) {
            if (s.charAt(i) == '(') left++;
            if (s.charAt(i) == ')') right++;
            if (left == right) {
                ans = Math.max(ans, 2*left);
            } else if(left > right) {
                left = 0;
                right = 0;
                
            }
        }
        return ans;
    }
}