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LeetCode 415. Add Strings

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Question

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

Note:

  • 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.

Explanation

得到两个用String表示的数的和。不能直接转成int再加，会有overflow的问题。所以一位一位地加，计算进位，用stringbuilder得到最后的结果。

Code

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public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        int i=num1.length()-1; int j=num2.length()-1;
        int a = 0; int b=0;
        while (i >=0 || j>=0 || carry >= 1) {
            if (i >= 0) {
                a = num1.toCharArray()[i--] - '0';
            } else {
                a = 0;
            }
            if (j >= 0) {
                b = num2.toCharArray()[j--] - '0';
            } else {
                b = 0;
            }
            int sum = a+b+carry;
            carry = sum/10;
            sb.insert(0,sum%10);
        }
        return sb.toString();
    }
}