[LeetCode]Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

思考：大数相加的思想。

class Solution {

public:

    string addBinary(string a, string b) {

        if(a.length()==0) return b;

        if(b.length()==0) return a;

        int len=a.length()<b.length()?a.length():b.length();

        if(len==a.length()) swap(a,b);

        int len1=a.length();

        int len2=b.length();

        int i;

        for(i=0;i<len;i++)

        {

            a[len1-1-i]=(a[len1-1-i]-'0')+(b[len2-1-i]-'0')+'0';

        }

        for(i=len1-1;i>0;i--)

        {

            if(a[i]>='2') 

            {

                a[i]=(a[i]-'0')%2+'0';

                a[i-1]=(a[i-1]-'0'+1)+'0';

            }

        }

        if(a[0]>='2') 

        {

            string ans(a,1,len1-1);

            if(a[0]=='2') return "10"+ans;

            else return "11"+ans;

        }

        else return a;

        

    }

};