Add two integers represented as strings and return the sum as new string

C++ solution :

class Solution {
public:
    string addBinary(string a, string b) {
       
        int lena = a.size() - 1;
        int lenb = b.size() - 1;
        string s = "";
        int sum = 0, carry = 0;
        while(lena >= 0 || lenb >= 0 || carry==1) {
            int n1 = lena >= 0 ? a[lena--] - '0' : 0;
            int n2 = lenb >= 0 ? b[lenb--] - '0' : 0;
            sum = n1^n2^carry;
            carry = n1&n2|n1&carry|n2&carry;
            char c = ((sum % 2) + '0');
            s = c + s;
        }
        return s;       
    }
};

or

class Solution {
public:
    string addBinary(string a, string b) {
       
        int lena = a.size() - 1;
        int lenb = b.size() - 1;
        string s = "";
        int sum = 0, carry = 0;
        while(lena >= 0 || lenb >= 0 || carry==1) {
            int n1 = lena >= 0 ? a[lena--] - '0' : 0;
            int n2 = lenb >= 0 ? b[lenb--] - '0' : 0;
            sum = n1 + n2 + carry;
            carry = sum / 2;
            char c = ((sum % 2) + '0');
            s = c + s;
        }
        return s;       
    }
};


C solution : 

char* addStrings(char* num1, char* num2) {
    int len1 = 0;
    int len2 = 0;
    int i,j;
    for(i=0;num1[i]!= NULL;i++)
        len1++;
    for(j=0;num2[j]!= NULL;j++)
        len2++;
   
    printf("len1 %d, len2 %d", len1, len2);
    int finalSize = (len1 > len2) ? len1:len2;
   
    char* result = malloc((finalSize+1)*sizeof(char));
    int k = finalSize - 1;
    int carry = 0;
    int carry_temp = 0;
    int sum = 0;
    while(len1 && len2) { //584, 18
        sum = (num1[len1 - 1] - '0') + (num2[len2 - 1] - '0');
        if (sum >= 10) {
            carry_temp = sum / 10;
            sum = sum % 10;
        }
        if((sum+carry)>=10) {
            sum = sum + carry;
            carry_temp = sum / 10;
            sum = sum % 10;
            carry = 0;
        }
        result[k] = ((sum + carry) +'0');
        carry = carry_temp;
        carry_temp = 0;
        len1--;
        len2--;
        k--;
    }
    while (len1) {
        sum = ((num1[len1-1] - '0') + carry);
        if (sum >= 10) {
            carry_temp = sum / 10;
            sum = sum % 10;
        }
        result[k] = (sum +'0');
        carry = carry_temp;
        carry_temp = 0;
        k--;
        len1--;
    }
   
    while (len2) {
        sum = ((num2[len2-1] - '0') + carry);
        if (sum >= 10) {
            carry_temp = sum / 10;
            sum = sum % 10;
        }
        result[k] = (sum +'0');
        carry = carry_temp;
        carry_temp = 0;
        k--;
        len2--;
    }

    if (carry) {
        char *result_new = malloc((finalSize+2)*sizeof(char));
        int l = 0;
       
        while(l < finalSize-1) {
            result_new[l+1] = result[l];
            l++;
        }
        result_new[l+1] = result[l];
        result_new[0] = carry + '0';
        result_new[finalSize+1] = NULL;
        return result_new;
    }
    result[finalSize] = NULL;
    return result;
}