32. Longest Valid Parentheses

    /*
     * 32. Longest Valid Parentheses 
     * 2016-4-18 by Mingyang
     * 那么这里我用stack里面装的是index而不是实际的括号
     * 实际代码如题，很简单
     */
   

public int longestValidParentheses(String s) {
    Stack<Integer> stack = new Stack<Integer>();
    int max=0;
    int left = -1;
    for(int j=0;j<s.length();j++){
        if(s.charAt(j)=='(') stack.push(j);            
        else {
            if (stack.isEmpty()) left=j;
            else{
                stack.pop();
                if(stack.isEmpty()) max=Math.max(max,j-left);
                else max=Math.max(max,j-stack.peek());
               }
            }
        }
    return max;
}