Valid Parentheses

parens.rs

pub struct Solution {}

impl Solution {
    pub fn is_valid(s: &str) -> bool {
        let mut num_open_seen = 0;
        for (i, c) in s.chars().enumerate() {
            let maybe_last_open = s.split_at(i)
                .0
                .chars()
                .rev()
                .filter(|c| match c {
                    &'{' | &'[' | &'(' => true,
                    _ => false,
                })
                .skip(num_open_seen)
                .next();
            match c {
                '}' => {
                    match maybe_last_open {
                        Some(last_open) => {
                            if last_open != '{' {
                                return false;
                            }
                        }
                        None => {
                            return false;
                        }
                    }
                    num_open_seen += 1;
                }
                ']' => {
                    match maybe_last_open {
                        Some(last_open) => {
                            if last_open != '[' {
                                return false;
                            }
                        }
                        None => {
                            return false;
                        }
                    }
                    num_open_seen += 1;
                }
                ')' => {
                    match maybe_last_open {
                        Some(last_open) => {
                            if last_open != '(' {
                                return false;
                            }
                        }
                        None => {
                            return false;
                        }
                    }
                    num_open_seen += 1;
                }
                // You might think we push onto a stack here
                _ => {}
            }
        }
        return true;
    }
}

fn main() {
    println!("{}", Solution::is_valid("(){}"));
}