求mex（最小未出现的自然数）

1.O(nlogn)

set<int> s;
for (int i = 0; i < n; ++i)
    s.insert(a[i]);

ll mex = 0;
while (s.count(mex))
    ++mex;

2.O(n)

vector<int> st(n + 10, 0);
for (int i = 0; i < n; ++i)
    if (a[i] <= n)
        st[a[i]] = 1;

int mex = 0;
while (st[mex])
    ++mex;

Max 与 Min

\(\max(a, b) = \frac{a + b + |a – b|}{2}\) \(\min(a, b) = \frac{a + b – |a – b|}{2}\)

上取整

\(\frac{a}{b} = \frac{a + b – 1}{b}\)

按位与

\(x|y=(x+y)-(x\&y)\)