USACO 2023 January Contest, Silver

Problem 3. Moo Route


Contest has ended.

Log in to allow submissions in analysis mode


Farmer Nhoj dropped Bessie in the middle of nowhere! At time $t=0$, Bessie is located at $x=0$ on an infinite number line. She frantically searches for an exit by moving left or right by $1$ unit each second. However, there actually is no exit and after $T$ seconds, Bessie is back at $x=0$, tired and resigned.

Farmer Nhoj tries to track Bessie but only knows how many times Bessie crosses $x=.5, 1.5, 2.5, \ldots, (N-1).5$, given by an array $A_0,A_1,\dots,A_{N-1}$ ($1\leq N \leq 10^5$, $1 \leq A_i \leq 10^6$, $\sum A_i\le 10^6$). Bessie never reaches $x>N$ nor $x<0$.

In particular, Bessie's route can be represented by a string of $T = \sum_{i=0}^{N-1} A_i$ $L$s and $R$s where the $i$th character represents the direction Bessie moves in during the $i$th second. The number of direction changes is defined as the number of occurrences of $LR$s plus the number of occurrences of $RL$s.

Please help Farmer Nhoj find any route Bessie could have taken that is consistent with $A$ and minimizes the number of direction changes. It is guaranteed that there is at least one valid route.

INPUT FORMAT (input arrives from the terminal / stdin):

The first line contains $N$. The second line contains $A_0,A_1,\dots,A_{N-1}$.

OUTPUT FORMAT (print output to the terminal / stdout):

Output a string $S$ of length $T = \sum_{i=0}^{N-1} A_i$ where $S_i$ is $L$ or $R$, indicating the direction Bessie travels in during second $i$. If there are multiple routes minimizing the number of direction changes, output any.

SAMPLE INPUT:

2
2 4

SAMPLE OUTPUT:

RRLRLL

There is only 1 valid route, corresponding to the route $0\to 1 \to 2 \to 1\to 2 \to 1\to 0$. Since this is the only possible route, it also has the minimum number of direction changes.

SAMPLE INPUT:

3
2 4 4

SAMPLE OUTPUT:

RRRLLRRLLL

There are 3 possible routes:

RRLRRLRLLL
RRRLRLLRLL
RRRLLRRLLL

The first two routes have 5 direction changes, while the last one has only 3. Thus the last route is the only correct output.

SCORING:

  • Inputs 3-5: $N\le 2$
  • Inputs 3-10: $T = A_0 + A_1 + \cdots + A_{N-1} \leq 5000$
  • Inputs 11-20: No additional constraints.

Problem credits: Brandon Wang and Claire Zhang

Contest has ended. No further submissions allowed.