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# Remove Invalid Parentheses LeetCode Solution

## Problem – Remove Invalid Parentheses

Given a string `s` that contains parentheses and letters, remove the minimum number of invalid parentheses to make the input string valid.

Return all the possible results. You may return the answer in any order.

Example 1:

``````Input: s = "()())()"
Output: ["(())()","()()()"]``````

Example 2:

``````Input: s = "(a)())()"
Output: ["(a())()","(a)()()"]``````

Example 3:

``````Input: s = ")("
Output: [""]``````

Constraints:

• `1 <= s.length <= 25`
• `s` consists of lowercase English letters and parentheses `'('` and `')'`.
• There will be at most `20` parentheses in `s`.

### Remove Invalid Parentheses LeetCode Solution in Java

``````public List<String> removeInvalidParentheses(String s) {
List<String> ans = new ArrayList<>();
remove(s, ans, 0, 0, new char[]{'(', ')'});
return ans;
}

public void remove(String s, List<String> ans, int last_i, int last_j,  char[] par) {
for (int stack = 0, i = last_i; i < s.length(); ++i) {
if (s.charAt(i) == par[0]) stack++;
if (s.charAt(i) == par[1]) stack--;
if (stack >= 0) continue;
for (int j = last_j; j <= i; ++j)
if (s.charAt(j) == par[1] && (j == last_j || s.charAt(j - 1) != par[1]))
remove(s.substring(0, j) + s.substring(j + 1, s.length()), ans, i, j, par);
return;
}
String reversed = new StringBuilder(s).reverse().toString();
if (par[0] == '(') // finished left to right
remove(reversed, ans, 0, 0, new char[]{')', '('});
else // finished right to left
}
``````

### Remove Invalid Parentheses LeetCode Solution in Python

``````def removeInvalidParentheses(self, s):
level = {s}
while True:
valid = []
for s in level:
try:
eval('0,' + filter('()'.count, s).replace(')', '),'))
valid.append(s)
except:
pass
if valid:
return valid
level = {s[:i] + s[i+1:] for s in level for i in range(len(s))}``````

### Remove Invalid Parentheses LeetCode Solution in C++

``````class Solution {
public:
vector<string> removeInvalidParentheses(string s) {
unordered_set<string> result;
int left_removed = 0;
int right_removed = 0;
for(auto c : s) {
if(c == '(') {
++left_removed;
}
if(c == ')') {
if(left_removed != 0) {
--left_removed;
}
else {
++right_removed;
}
}
}
helper(s, 0, left_removed, right_removed, 0, "", result);
return vector<string>(result.begin(), result.end());
}
private:
void helper(string s, int index, int left_removed, int right_removed, int pair, string path, unordered_set<string>& result) {
if(index == s.size()) {
if(left_removed == 0 && right_removed == 0 && pair == 0) {
result.insert(path);
}
return;
}
if(s[index] != '(' && s[index] != ')') {
helper(s, index + 1, left_removed, right_removed, pair, path + s[index], result);
}
else {
if(s[index] == '(') {
if(left_removed > 0) {
helper(s, index + 1, left_removed - 1, right_removed, pair, path, result);
}
helper(s, index + 1, left_removed, right_removed, pair + 1, path + s[index], result);
}
if(s[index] == ')') {
if(right_removed > 0) {
helper(s, index + 1, left_removed, right_removed - 1, pair, path, result);
}
if(pair > 0) {
helper(s, index + 1, left_removed, right_removed, pair - 1, path + s[index], result);
}
}
}
}
};
``````
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