Leetcode add to array form of integer

leetcode-add-to-array-form-of-integer.md

Original problem: https://leetcode.com/problems/add-to-array-form-of-integer/description/

Posted solution: https://leetcode.com/problems/add-to-array-form-of-integer/solutions/4212467/rust-add-digits-by-digits-in-an-array-to-sum-very-large-numbers/

Intuition

I thought it was just a simple string conversion and simple arithmetic but it's more complicated than that.

Given a very large number, simply add operation breaks even on i128 in rust.

Approach

Map both num and k into an array then add digit by digit from right to left. If sum is >= 10, carry over the excess then set the value of the current digit.

If there are excess carry over at the end of the operation, add more digit to the right.

fn solution_add_to_array_form_of_integer() {
    assert_eq!(add_to_array_form_of_integer(vec![1, 2, 0, 0], 34), vec![1, 2, 3, 4]);
    assert_eq!(add_to_array_form_of_integer(vec![2, 7, 4], 181), vec![4, 5, 5]);
    assert_eq!(add_to_array_form_of_integer(vec![2, 1, 5], 806), vec![1, 0, 2, 1]);
    assert_eq!(add_to_array_form_of_integer(vec![9, 9, 9, 9, 9, 9, 9, 9, 9, 9], 1), vec![1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
    assert_eq!(
    add_to_array_form_of_integer(vec![3,5,8,6,9,7,8,3,8,5,4,1,6,7,4,1,0,1,7,7,1,5,3,2,9,3,4,1,0,5,8,6,9,9,4,8,7,0,2,8,2,4,7,0,4,4,3,7,2,2], 142),
    vec![3,5,8,6,9,7,8,3,8,5,4,1,6,7,4,1,0,1,7,7,1,5,3,2,9,3,4,1,0,5,8,6,9,9,4,8,7,0,2,8,2,4,7,0,4,4,3,8,6,4]
);
}

fn add_to_array_form_of_integer(num: Vec<i32>, k: i32) -> Vec<i32> {
    // Convert k into an array of digits too
    // Add num and k digit by digit from right to left
    // Carry over values to the next digit if >= 10
    // Add more digit if necessary

    // Reverse the numbers
    let mut numbers = num.clone();
    numbers.reverse();

    // Convert k to array and reverse it too
    let mut ks: Vec<i32> = Vec::new();
    for digit_char in k.to_string().chars() {
        let digit: i32 = digit_char.to_string().parse().unwrap();
        ks.push(digit);
    }
    ks.reverse();

    // Start adding digit by digit
    let mut carry_over: i32 = 0;
    let mut counter: usize = 0;
    let mut result: Vec<i32> = Vec::new();

    while counter < numbers.len() || counter < ks.len() {
        let a: i32 = match numbers.get(counter) {
            Some(val) => *val,
            None => 0,
        };
        let b: i32 = match ks.get(counter) {
            Some(val) => *val,
            None => 0,
        };
        let sum: i32 = a + b + carry_over;
        if sum >= 10 {
            let diff = sum - 10;
            carry_over = 1;
            result.push(diff);
        } else {
            carry_over = 0;
            result.push(sum);
        }
        counter += 1;
    }

    if carry_over > 0 {
        result.push(carry_over);
    }

    // Reverse it back
    result.reverse();
    return result;
}