[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;
        
    }
};