SRM 553 DIV2

250.

解三元一次方程组。

class PlatypusDuckAndBeaver
{
    public:
    int minimumAnimals(int a, int b, int c)
    {
        int k = (2*b+4*c-a)/2;
        int x = b - k;
        int y = c - k;
        return x + y + k;
    }
};

500.

二分答案。

typedef long long LL;
class Suminator
{
    public:
    LL work(vector<int>f,int want)
    {
        stack<LL>S;
        S.push(0);
        for(int i=0;i<f.size();i++)
        {

            if(f[i] == 0)
            {
                LL tmp = 0;
                for(int j =S.size(),k=0;j>0&&k<2;k++,j--){
                    tmp += (LL)S.top();
                    S.pop();
                }
                S.push(tmp);
            }
            else S.push((LL)f[i]);
        }
        return S.top() - (LL)want;
    }
    int findMissing(vector <int> f, int want)
    {
        int n = f.size();
        int x=0;
        for(int i=0;i<n;i++)
        {
            if(f[i] == -1)
            {
                x = i;
                break;
            }
        }
        f[x]=0;
        if(work(f,want) == 0 )return 0;
        int l = 1,r = 1000000000;
        while(l<=r)
        {
            int m = (l + r)>>1;
            f[x] = m;
            LL tmp = work(f,want);
            if(tmp == 0)return m;
            else if(tmp > 0)r = m - 1;
            else l = m + 1;
        }
        return -1;
    }
};