LeetCode [415] Add Strings

 415. Add Strings

Easy

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

Note:

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

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class Solution { public String addStrings(String num1, String num2) { int n1 = num1.length(), n2 = num2.length(); StringBuilder sb = new StringBuilder(); int i = n1-1, j = n2-1; int c = 0; while(i>=0 || j>=0 || c>0){ int s = c; if(i>=0) s += (num1.charAt(i)-'0'); if(j>=0) s += (num2.charAt(j)-'0'); c = s/10; s = s%10; char ch = (char)(s+'0'); sb.append(ch); i--; j--; } return sb.reverse().toString(); } }