【leetcode】Add Binary

Add Binary 

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

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

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先补全字符串，从末尾开始加，每次计算当前位和进位即可

class Solution {
public:
    char addBit(char a,char b,char &c)
    {
        if(a==b&&a=='1')
        {
            char ret;
            ret=c;
            c='1';
            return ret;
        }
        else if(a==b&&a=='0')
        {
            
            if(c=='0')
            {
                return '0';
            }
            else
            {
                c='0';
                return '1';
            }
        }
        else
        {
            if(c=='1')
               return '0';
            else
               return '1';
            
        }
    }
    
    string addBinary(string a, string b) {
        
        int na=a.length()-1;
        int nb=b.length()-1;
        
        if(na!=nb)
        {
            string tmp(abs(na-nb),'0');
            if(na>nb)
                b=tmp+b;
            else
                a=tmp+a;
        }
        
        int i=a.length()-1;
        string result(a.length(), '1');
        char c='0';
        
        while(i>=0)
        {
            result[i]=addBit(a[i],b[i],c);
            i--;
        }
        if(c=='0')
        {
            return result;
        }
        else
        {
            return '1'+result;
        }
    }
};