BigInteger.java

public class BigInteger {
    DigitNode front;

    private static class DigitNode {
        int digit;
        DigitNode next;
        public DigitNode(int digit) {
            this.digit = digit;
            this.next = null;
        }
        public DigitNode(int digit, DigitNode next) {
            this.digit = digit;
            this.next = next;
        }
        public String toString(){
            DigitNode cur = this;
            String out = "";
            while (cur != null) {
               out += "(" + cur.digit + ") -> ";
               cur = cur.next;
            }
            return out;
        }
    }

    public String toString() {
        return this.front.toString();
    }

    // public BigInteger(String input) {
    //     this.front = new DigitNode(Character.getNumericValue(input.charAt(0)));
    //     DigitNode cur = this.front;
    //     for (int i = 1; i < input.length(); i++) {
    //         cur.next = new DigitNode(Character.getNumericValue(input.charAt(i)));
    //         cur = cur.next;
    //     }
    // }

    public DigitNode createBackwards(String num, DigitNode last) {
        if (num.length() == 0) {
            return last;
        } else {
            return createBackwards(num.substring(1), new DigitNode(Character.getNumericValue(num.charAt(0)), last));
        }
    }

    public BigInteger(String input) {
        this.front = this.createBackwards(input.substring(1), new DigitNode(Character.getNumericValue(input.charAt(0))));
    }

    public static void main (String[] args) {
        BigInteger b = new BigInteger("12345");
        System.out.println(b);
    }
}