Add Binary

主要拿来练C语言语法，代码不是很好看

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

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

char* addBinary(char* a, char* b) {
    char *pa = a;
    char *pb = b;
    int a_len = 0;
    int b_len = 0;
    while(*pa != '\0'){
        ++pa;
        ++a_len;
    }
    while(*pb != '\0'){
        ++pb;
        ++b_len;
    }
    int ai,bi;
    int len = (a_len>b_len?a_len:b_len)+2;
    char *res = (char*)malloc(len*sizeof(char));
    char carry_bit = '0';
    res[len-1] = '\0';
    int res_i = len-2;
    --pa;
    --pb;
    for (ai = a_len,bi = b_len; ai > 0 && bi > 0; --ai,--bi,--pa, --pb){
        res[res_i] = (( (*pa-'0') ^ (*pb-'0') ^ (carry_bit-'0') ) )+'0';
        
        if(*pa == '0'){
            carry_bit = ( (*pb-'0') & (carry_bit-'0') ) + '0';
        }else{
            carry_bit = ( (*pb-'0') || (carry_bit-'0') ) + '0';
        }
        --res_i;
    }
    while(ai>0){
        res[res_i] = (( (*pa-'0') ^ (carry_bit-'0') ) )+'0';
        carry_bit = ( (*pa-'0') & (carry_bit-'0')) + '0';
        --res_i;
        --ai;
        --pa;
    }
    while(bi>0){
        res[res_i] = (( (*pb-'0') ^ (carry_bit-'0') ) )+'0';
        carry_bit = ( (*pb-'0') & (carry_bit-'0')) + '0';
        --res_i;
        --bi;
        --pb;
    }
    if(carry_bit == '1'){
        res[res_i] = carry_bit;
    }else{
        for(res_i = 1; res_i < len; ++res_i){
            res[res_i-1] = res[res_i];
        }
        res[len-1] = '\0';
    }
    return res;
}