415. Add Strings (计算由两个字符串表示的数字相加之和）

Given two non-negative numbers 

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.

public class Solution {
public String addStrings(String num1, String num2) {
int n1, n2, sum, status = 0,minLen,maxLen;
if(num1.length()>num2.length()){
minLen = num2.length();
maxLen = num1.length();
int i = maxLen - minLen;
while(i-->0)
num2 = "0"+num2;
}else{
minLen = num1.length();
maxLen = num2.length();
int i = maxLen - minLen;
while(i-->0)
num1 = "0"+num1;
}
StringBuilder sb = new StringBuilder();

for (int i = num1.length()-1; i >= 0; i--) {
n1 = num1.charAt(i) - '0';
n2 = num2.charAt(i) - '0';
sum = n1 + n2 +status;
if (sum < 10) {
sb.insert(0,(char) ('0' + sum));
status = 0;
} else {
sb.insert(0,(char) ('0' + sum % 10));
status = 1;
}
}if(status==1)
sb.insert(0, '1');
return sb.toString();
}
}