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