upper_bound and lower_bound
ref: https://hackmd.io/@sa072686/cp/%2F%40sa072686%2FHk8wnUcWB
- upper_bound: first
i
thata[i] > k
- lower_bound: first
i
thata[i] >= k
ref: https://hackmd.io/@sa072686/cp/%2F%40sa072686%2FHk8wnUcWB
i
that a[i] > k
i
that a[i] >= k