public class Solution {    public List<String> generateParenthesis(int n) {        List<String> result = new ArrayList<String>();        Stack<Integer> numStack = new Stack<Integer>();        int nowNum = n;        boolean nowNumPoped = false;        while (!(nowNumPoped && nowNum == n)) {            if (!nowNumPoped) {                if (nowNum>0) {                    numStack.push(nowNum);                    if (numStack.size()+nowNum == n+1)                        nowNum--;                } else {                    result.add(generateString(numStack,n));                    nowNum = numStack.pop();                    nowNumPoped = true;                }            } else {                nowNum--;                nowNumPoped = false;            }        }        return result;    }        public String generateString(Stack<Integer> numStack, int n) {        int rightParentheses = 0;        StringBuffer resultString = new StringBuffer();        for (int i=0; i<n; i++) {            resultString.append("(");            int tmpIndex = n-i-1;            if (tmpIndex<numStack.size()) {                while (rightParentheses<numStack.get(tmpIndex)) {                    resultString.append(")");                    rightParentheses++;                }            }        }        return resultString.toString();    }}