最长有效括按号长度（利用栈解决java语言）

public static int longestValidParentheses(String s){
		Stack<Character> stack = new Stack<Character>();
		int count = 0;
		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='(')
				stack.push(s.charAt(i));
			if(!stack.isEmpty()&&s.charAt(i)==')'){
				stack.pop();
				count+=2;
			}
		}
		return count;
	}