示例 1：
输入：s = "()())()"输出：["(())()","()()()"]
来源：力扣（LeetCode）链接：https://leetcode-cn.com/problems/remove-invalid-parentheses著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

class Solution {    private int len;    private char[] charArray;    private Set<String> validExpressions = new HashSet<>();

    public List<String> removeInvalidParentheses(String s) {        this.len = s.length();        this.charArray = s.toCharArray();        int leftRemove = 0;        int rightRemove = 0;        for (char ch : charArray) {            if (ch == '(') {                leftRemove++;            } else if (ch == ')') {                if (leftRemove == 0) {                    rightRemove++;                } else if (leftRemove > 0) {                    leftRemove--;                }            }        }
        StringBuilder path = new StringBuilder();        dfs(0, 0, 0, leftRemove, rightRemove, path);        return new ArrayList<>(this.validExpressions);    }
    public void dfs(int index, int leftCount, int rightCount, int leftRemove, int rightRemove, StringBuilder path) {        if (index == len) {            if (leftRemove == 0 && rightRemove == 0) {                validExpressions.add(path.toString());            }            return;        }
        char character = charArray[index];
        if (character == '(' && leftRemove > 0) {            dfs(index + 1, leftCount, rightCount, leftRemove - 1, rightRemove, path);        }
        if (character == ')' && rightRemove > 0) {            dfs(index + 1, leftCount, rightCount, leftRemove, rightRemove - 1, path);        }
        path.append(character);
        if (character != '(' && character != ')') {            dfs(index + 1, leftCount, rightCount, leftRemove, rightRemove, path);        } else if (character == '(') {            dfs(index + 1, leftCount + 1, rightCount, leftRemove, rightRemove, path);        } else if (rightCount < leftCount) {            dfs(index + 1, leftCount, rightCount + 1, leftRemove, rightRemove, path);        }
        path.deleteCharAt(path.length() - 1);    }}