leetcode 67. Add Binary

67. 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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class Solution {public:    string addBinary(string a, string b)     {        int la = a.size();        int lb = b.size();        if (la == 0)            return b;        else if (lb == 0)            return a;                  int k = max(la, lb) + 1;                for (int i = la; i < k; i++)            a = "0" + a;                   for (int i = lb; i < k; i++)            b = "0" + b;                   int flag = 0;        for(int i = k - 1; i >= 0; i--)        {            if (flag == 0)            {                if(a[i]=='1' && b[i]=='1')                 {                     a[i] = '0';                    flag = 1;                }                   else if ((a[i]=='1' && b[i]=='0') || (a[i]=='0' && b[i]=='1'))                    a[i] = '1';            }            else            {                if((a[i]=='1' && b[i]=='0') || (a[i]=='0' && b[i]=='1'))                {                    a[i] = '0';                }                else if(a[i]=='0' && b[i]=='0')                 {                    a[i] = '1';                    flag = 0;                }            }        }        return (a[0] == '1') ? a : a.substr(1, a.size()-1);    }};