Solution.rs

impl Solution {
    pub fn longest_valid_parentheses(s: String) -> i32 {
        let mut ans = 0;
        let mut f = vec![0; s.len() + 1];
        for i in 2..=s.len() {
            if
                s
                    .chars()
                    .nth(i - 1)
                    .unwrap() == ')'
            {
                if
                    s
                        .chars()
                        .nth(i - 2)
                        .unwrap() == '('
                {
                    f[i] = f[i - 2] + 2;
                } else if
                    (i as i32) - f[i - 1] - 1 > 0 &&
                    s
                        .chars()
                        .nth(i - (f[i - 1] as usize) - 2)
                        .unwrap() == '('
                {
                    f[i] = f[i - 1] + 2 + f[i - (f[i - 1] as usize) - 2];
                }
                ans = ans.max(f[i]);
            }
        }
        ans
    }
}