[LintCode] Big Integer Addition

i_garfileo 發布于2019-08-16 10:59 / 2887人閱讀

Problem

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Notice

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

Example

Given num1 = "123", num2 = "45"
return "168"

Solution

public class Solution {
    /*
     * @param num1: a non-negative integers
     * @param num2: a non-negative integers
     * @return: return sum of num1 and num2
     */
    public String addStrings(String num1, String num2) {
        //start from adding the last digits of num1, num2:
        //if the current sum > 10, save 1 in `carry`,
        //add to the front of StringBuilder sb
        //... doing this till both indice less than 0
        
        int i = num1.length()-1, j = num2.length()-1, carry = 0, curSum = 0;
        StringBuilder sb = new StringBuilder();
        while (i >= 0 || j >= 0 || carry == 1) {
            //Integer.valueOf(String.valueOf(char)) is to remind me that the value of char is mapped to the decimal value in ascii 
            int curNum1 = i >= 0 ? Integer.valueOf(String.valueOf(num1.charAt(i))) : 0;
            int curNum2 = j >= 0 ? Integer.valueOf(String.valueOf(num2.charAt(j))) : 0;
            int sum = carry + curNum1 + curNum2;
            curSum = sum % 10; carry = sum/10;
            sb.insert(0, curSum);
            i--; j--;
        }
        return sb.toString();
    }
}

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