Hi all, Atcoder Beginner Contest 143 was today. I wrote an unofficial English editorial. Hope it helps!

A: Curtain

The curtains can cover a maximum of $$$2B$$$. So either they don't cover the whole window and the remainder is $$$A-2B$$$, or they do and the remainder is $$$0$$$. So the answer is $$$\max(0, A-2B)$$$.

Runtime: $$$\mathcal{O}(N)$$$.

int a = in.nextInt(), b = in.nextInt();
out.println(Math.max(0, a - 2 * b));

B: TAKOYAKI FESTIVAL 2019

We can simply loop through every pair (taking care not to double count) and add the health points restored.

Runtime: $$$\mathcal{O}(N^2)$$$.

int n = in.nextInt();
int[] d = in.readIntArray(n);

long answer = 0;
for (int i = 0; i < n; i++) {
    for (int j = i + 1; j < n; j++) {
        answer += d[i] * d[j];
    }
}

out.println(answer);

C: Slimes

We have to count the number of "runs" of consecutive slimes of the same color. We can do this by looping through the array and checking whether the current slime continues the run, then incrementing our answer when we reach the end of a run.

Runtime: $$$\mathcal{O}(N)$$$.

int n = in.nextInt();
char[] s = in.next().toCharArray();

int answer = 0;
for (int i = 0, j = 0; i < n; i = j) {
    while (j < n && s[i] == s[j])
        j++;

    answer++;
}
out.println(answer);

D: Triangles

The total number of (unordered) triples of sticks is $$$\binom{n}{3} = n(n-1)(n-2)/3$$$. I think it's simplest to count the number of triples that cannot form a triangle, and subtract them. Let's first sort the input array, then if we have a triple $$$(i,j,k)$$$ with $$$i < j < k$$$, we also know $$$L_i \le L_j \le L_k$$$.

Runtime: $$$\mathcal{O}(N^2 \log N)$$$.

E: Travel by Car

Runtime: $$$\mathcal{O}()$$$.

F: Distinct Numbers

Runtime: $$$\mathcal{O}(N \log {N})$$$.

Thanks for reading! Let me know if you have any questions or feedback for me.