Longest Valid Parentheses

public class Solution {
    public int longestValidParentheses(String s) {
		// Start typing your Java solution below
		// DO NOT write main() function
		int len = s.length();
		int max = 0;
		Stack<Integer> stack = new Stack<Integer>();
		for(int i = 0; i < len; i++){
			if(s.charAt(i) == '(')
				stack.push(i);
			else{
				if(!stack.empty() && s.charAt(stack.peek()) == '('){
					stack.pop();
					if(stack.empty())
						max = Math.max(max, i + 1);
					else{
						max = Math.max(max, i - stack.peek());
					}
				}
				else
					stack.push(i);
			}
		}
		return max;
	}
}

Same idea. Using arrays instead of a stack.

public class Solution {
    public int longestValidParentheses(String s) {
		// Start typing your Java solution below
		// DO NOT write main() function
		if (s == null || s.length() == 0)
			return 0;
		int l = s.length();
		int dp[] = new int[l];
		int max = 0, j;

		for (int i = 1; i < l; i++) {
			if (s.charAt(i) == ')') {
				j = i - 1 - dp[i - 1];
				if (j >= 0 && s.charAt(j) == '(')
					dp[i] = i - j + 1 + ((j > 0) ? dp[j - 1] : 0);
			}

			if (dp[i] > max)
				max = dp[i];
		}
		return max;
	}
}