Ferris wheel

$n$ children want to ride a Ferris wheel; the weight of child $i$ is $p_i$. Each gondola takes one or two children, and the total weight in a gondola must not exceed $x$. What is the minimum number of gondolas needed for all children?

The first line contains $n$ and $x$ ($1 \le n \le 2 \cdot 10^5$, $1 \le x \le 2 \cdot 10^9$). The second line contains $n$ integers $p_i$ ($1 \le p_i \le x$).

A single number — the minimum number of gondolas.

Sort the weights and keep two pointers: the lightest ($l$) and the heaviest ($r$) child. The heaviest child takes some gondola anyway; if the lightest one fits with them, seat them together (moving $l$ too), otherwise the heavy child rides alone. $r$ decreases at every step, so this is $O(n)$.

$p_l + p_r$ reaches $4 \cdot 10^9$, which does not fit into a 32-bit int.

Source: CSES, problem 1090.