conversion https://leetcode.com/submissions/detail/266243610/ to rust

to_rust.rs

impl Solution {
    pub fn first(t : i32 , n: i32) -> i32 {
        2*(n-t) - 1
    }
    pub fn second(t : i32) -> i32 {
        2*t - 3
    }
    pub fn convert(s: String, num_rows: i32) -> String {
         let mut ans = String::new();
         let indexCounter = 0;
         let len = s.len();
         if num_rows==1 || len == 1 || len <= num_rows {
             s
         }

        let mut i = 1;
        while i < num_rows {
            let mut index = i;
            let flag = false;

            while index <= len {
                ans.push(s.chars().nth(index-1).unwrap());
                let gap1 = first (&i, &num_rows);
                let gap2 = second(i);

                if i == num_rows {
                    index = index + gap2 + 1;
                }
                else if i == 1 {
                    index = index + gap1 + 1;
                }
                else {
                    if flag == 0 {
                        index = index + gap1 + 1;
                        flag = true;
                    }
                    else {
                        index = index + gap2 + 1;
                        flag = false;
                    }
                }
            }
            i = i + 1;
        }
        ans
    }
}