給定兩個(gè)二進(jìn)制字符串，返回他們的和（用二進(jìn)制表示）。
輸入為非空字符串且只包含數(shù)字 1 和 0。

示例 1:
輸入: a = "11", b = "1"
輸出: "100"
示例 2:
輸入: a = "1010", b = "1011"
輸出: "10101"

C

逆向遍歷
記錄標(biāo)記位，將標(biāo)記位與兩個(gè)字符串的對應(yīng)位相加，分析和，確定當(dāng)前位的值和是否需要進(jìn)位。

#define max(a, b) ((a) > (b) ? (a) : (b))
char * addBinary(char * a, char * b){
    if(a == NULL || *a == NULL)
        return b;
    if(b == NULL || *b == NULL)
        return a;
    int flag = 0;
    int len1 = strlen(a), len2 = strlen(b);
    int len = max(len1, len2) + 2;
    char* result = (char*)malloc(len * sizeof(char));
    result[len - 1] = '\0';
    int i = 0, j = 0, index = len - 2;
    while(len1 || len2 || flag) {
        int t = flag;
        if(len1)
            t += (a[--len1] - '0');
        if(len2)
            t += (b[--len2] - '0');
        flag = t / 2;
        result[index--] = '0' + t % 2;
    }
    if(index == 0) {
        char* temp = (char*)malloc((len - 1) * sizeof(char));
        memcpy(temp, result + 1, (len - 1) * sizeof(char));
        free(result);
        return temp;
    }
    return result;
}