// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

int firstBadVersion(int n) {
    //要減少呼叫api應快速查詢，採用二分法
    int left, right, mid;
    left = 1;
    right = n;
    while(left <= right){
        mid = left + (right - left) / 2;
        if(isBadVersion(mid))
            right = mid - 1;
        else
            left = mid + 1;   
    }
    return left;
}