SumOfTwoBigNumbers.java

package com.examplehub.maths;

import java.math.BigDecimal;

public class SumOfTwoBigNumbers {

  /**
   * Calculate sum of two big numbers.
   *
   * @param s1 the first number.
   * @param s2 the second number.
   * @return sum of the given two numbers.
   */
  public static String sum(String s1, String s2) {
    BigDecimal numberOne = new BigDecimal(s1);
    BigDecimal numberTwo = new BigDecimal(s2);
    BigDecimal sum = numberOne.add(numberTwo);
    return sum.toString();
  }

  /**
   * Calculate sum of two big numbers manually.
   *
   * @param s1 the first number.
   * @param s2 the second number.
   * @return sum of the given two numbers.
   */
  public static String sumOf(String s1, String s2) {
    char[] bytes1 = s1.toCharArray();
    char[] bytes2 = s2.toCharArray();
    int i1 = bytes1.length - 1;
    int i2 = bytes2.length - 1;
    int carry = 0;

    StringBuilder builder = new StringBuilder();
    while (i1 >= 0 || i2 >= 0) {
      int sum = carry + (i1 != -1 ? bytes1[i1] - '0' : 0) + (i2 != -1 ? bytes2[i2] - '0' : 0);
      carry = sum / 10;
      builder.append(sum % 10);
      if (i1 >= 0) {
        i1--;
      }
      if (i2 >= 0) {
        i2--;
      }
    }
    return carry != 0 ? builder.append(carry).reverse().toString() : builder.reverse().toString();
  }
}