Add Binary

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

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

思路： 对齐， 计算当前位和进位；

如果位较长，让我想到了先行进位加法器，不过这里的计算不能并行，也无什么效果。

    //add Binary  
    class Solution {  
    public:  
        string addBinary(string a, string b) {  
            // Start typing your C/C++ solution below  
            // DO NOT write int main() function  
                string c;  
                int flag=0;  
                int lena = a.size();  
                int lenb = b.size();  
                int len = abs(lena-lenb);  
                string append(len,'0');  
                if(lena>lenb){  
                    b = append + b;  
                    c.resize(lena,'0');  
                }else{  
                    a = append + a;  
                    c.resize(lenb,'0');  
                }  
                for(int j=c.size()-1;j>=0;j--){  
                    int current = (a[j]-'0') ^(b[j]-'0') ^flag;  
                    if((a[j]-'0') +(b[j]-'0') +flag >1)  
                        flag = 1;  
                    else   
                        flag = 0;  
                    c[j] = current+'0';  
                }  
                if(flag == 1)  
                    c = '1'+ c;  
                  
                return c;  
                  
        }  
    };  

我自己写了C的，改了好多次才成功

char *addBinary(char *a, char *b) {
    int flag = 0,temp,num;
    int i = 0;
    int alen = strlen(a)-1;
    int blen = strlen(b)-1;
    char *c =(char*) malloc(alen>blen? alen + 1 : blen + 1);
    while(alen >= 0 && blen >= 0){
        num = (a[alen--]-'0')+(b[blen--]-'0')+flag;
        flag = num/2;
        num = num%2;
        c[i++] = (char)(num + '0');
    }
    while(alen >=0){
        int num = a[alen--] - '0' + flag;
        flag = num / 2;
        num %= 2;
        c[i++] = (char)(num + '0');
    }
    while(blen >=0){
        int num = b[blen--] - '0' + flag;
        flag = num / 2;
        num %= 2;
        c[i++] = (char)(num + '0');
    }
    if(flag > 0){
    c[i++] = (char)(flag + '0');
    }
    c[i] = '\0';
    for(i = 0; i<(strlen(c)/2); i++){
        temp = c[i];
        c[i] = c[strlen(c)-i-1];
        c[strlen(c)-i-1] = temp;
    }
    return c;
    
}

其中c[i++]不能写成*c = (char)(num + '0'); c++; 因为遇到"0"和"0",给*c的是0，相当于截止符'\0',最后输出的是"".一直报错。