415. Add Strings

Given two non-negative numbers 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.

两个串的长度分别为L1、L2，分两段进行相加

第一段0到min(L1-1,L2-1)，第二段min(L1,L2)到max(L1-1,L2-1)，两段加完之后如果还有进位，需要继续补位数

public class Solution {
    public static String addStrings(String num1, String num2)
	{
		int len1=num1.length();
		int len2=num2.length();
		int carry=0;
		int minlen=Math.min(len1, len2);
		int cnt=0;
		StringBuilder sb=new StringBuilder();
		int i,j;
		for(i=len1-1,j=len2-1;cnt<minlen;i--,j--,cnt++)
		{
			int sum=(num1.charAt(i)-'0')+(num2.charAt(j)-'0')+carry;
			carry=sum/10;
			sum=sum%10;
			sb.append(sum);
		}
		if(len1>=len2)
			for(;i>=0;i--)
			{
				int sum=num1.charAt(i)-'0'+carry;
				carry=sum/10;
				sum%=10;
				sb.append(sum);
			}
		else {
			for(;j>=0;j--)
			{
				int sum=num2.charAt(j)-'0'+carry;
				carry=sum/10;
				sum%=10;
				sb.append(sum);
			}
		}
		if(carry>0)
			sb.append(carry);
		return sb.reverse().toString();
	}
}