题目：

题解：

class Solution {
    public List<String> removeInvalidParentheses(String s) {
        int lremove = 0;
        int rremove = 0;
        List<Integer> left = new ArrayList<Integer>();
        List<Integer> right = new ArrayList<Integer>();
        List<String> ans = new ArrayList<String>();
        Set<String> cnt = new HashSet<String>();

        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                left.add(i);
                lremove++;
            } else if (s.charAt(i) == ')') {
                right.add(i);
                if (lremove == 0) {
                    rremove++;
                } else {
                    lremove--;
                }
            }
        }

        int m = left.size();
        int n = right.size();
        List<Integer> maskArr1 = new ArrayList<Integer>();
        List<Integer> maskArr2 = new ArrayList<Integer>();
        for (int i = 0; i < (1 << m); i++) {
            if (Integer.bitCount(i) != lremove) {
                continue;
            }
            maskArr1.add(i);
        }
        for (int i = 0; i < (1 << n); i++) {
            if (Integer.bitCount(i) != rremove) {
                continue;
            }
            maskArr2.add(i);
        }
        for (int mask1 : maskArr1) {
            for (int mask2 : maskArr2) {
                if (checkValid(s, mask1, left, mask2, right)) {
                    cnt.add(recoverStr(s, mask1, left, mask2, right));
                }
            }
        }
        for (String v : cnt) {
            ans.add(v);
        }

        return ans;
    }

    private boolean checkValid(String str, int lmask, List<Integer> left, int rmask, List<Integer> right) {
        int pos1 = 0;
        int pos2 = 0;
        int cnt = 0;

        for (int i = 0; i < str.length(); i++) {
            if (pos1 < left.size() && i == left.get(pos1)) {
                if ((lmask & (1 << pos1)) == 0) {
                    cnt++;
                }
                pos1++;
            } else if (pos2 < right.size() && i == right.get(pos2)) {
                if ((rmask & (1 << pos2)) == 0) {
                    cnt--;
                    if (cnt < 0) {
                        return false;
                    }
                }
                pos2++;
            }
        }

        return cnt == 0;
    }

    private String recoverStr(String str, int lmask, List<Integer> left, int rmask, List<Integer> right) {
        StringBuilder sb = new StringBuilder();
        int pos1 = 0;
        int pos2 = 0;

        for (int i = 0; i < str.length(); i++) {
            if (pos1 < left.size() && i == left.get(pos1)) {
                if ((lmask & (1 << pos1)) == 0) {
                    sb.append(str.charAt(i));
                }
                pos1++;
            } else if (pos2 < right.size() && i == right.get(pos2)) {
                if ((rmask & (1 << pos2)) == 0) {
                    sb.append(str.charAt(i));
                }
                pos2++;
            } else {
                sb.append(str.charAt(i));
            }
        }

        return sb.toString();
    }
}