LeetCode 1249. Minimum Remove to Make Valid Parentheses

Problem Statement

use std::collections::HashSet;

impl Solution {
    pub fn min_remove_to_make_valid(s: String) -> String {
        let mut stack = vec![];
        let mut remove = vec![];
        for i in 0..s.len() {
            let c = s.chars().nth(i).unwrap();
            if c == '(' {
                stack.push(i);
            } else if c == ')' {
                if !stack.is_empty() {
                    stack.pop();
                } else {
                    remove.push(i);
                }
            }
        }

        while !stack.is_empty() {
            remove.push(stack.last().unwrap().clone());
            stack.pop();
        }

        let mut ans = "".to_string();
        let set : HashSet<usize> = remove.into_iter().collect();
        for i in 0..s.len() {
            if !set.contains(&i) {
                ans.push(s.chars().nth(i).unwrap());
            }
        }
        return ans;
    }
}