3015. Count the Number of Houses at a Certain Distance I
Description
You are given three positive integers n
, x
, and y
.
In a city, there exist houses numbered 1
to n
connected by n
streets. There is a street connecting the house numbered i
with the house numbered i + 1
for all 1 <= i <= n - 1
. An additional street connects the house numbered x
with the house numbered y
.
For each k
, such that 1 <= k <= n
, you need to find the number of pairs of houses (house1, house2)
such that the minimum number of streets that need to be traveled to reach house2
from house1
is k
.
Return a 1-indexed array result
of length n
where result[k]
represents the total number of pairs of houses such that the minimum streets required to reach one house from the other is k
.
Note that x
and y
can be equal.
Example 1:
![](https://spcdn.pages.dev/leetcode/problems/3015.Count%20the%20Number%20of%20Houses%20at%20a%20Certain%20Distance%20I/images/example2.png)
Input: n = 3, x = 1, y = 3 Output: [6,0,0] Explanation: Let's look at each pair of houses: - For the pair (1, 2), we can go from house 1 to house 2 directly. - For the pair (2, 1), we can go from house 2 to house 1 directly. - For the pair (1, 3), we can go from house 1 to house 3 directly. - For the pair (3, 1), we can go from house 3 to house 1 directly. - For the pair (2, 3), we can go from house 2 to house 3 directly. - For the pair (3, 2), we can go from house 3 to house 2 directly.
Example 2:
![](https://spcdn.pages.dev/leetcode/problems/3015.Count%20the%20Number%20of%20Houses%20at%20a%20Certain%20Distance%20I/images/example3.png)
Input: n = 5, x = 2, y = 4 Output: [10,8,2,0,0] Explanation: For each distance k the pairs are: - For k == 1, the pairs are (1, 2), (2, 1), (2, 3), (3, 2), (2, 4), (4, 2), (3, 4), (4, 3), (4, 5), and (5, 4). - For k == 2, the pairs are (1, 3), (3, 1), (1, 4), (4, 1), (2, 5), (5, 2), (3, 5), and (5, 3). - For k == 3, the pairs are (1, 5), and (5, 1). - For k == 4 and k == 5, there are no pairs.
Example 3:
![](https://spcdn.pages.dev/leetcode/problems/3015.Count%20the%20Number%20of%20Houses%20at%20a%20Certain%20Distance%20I/images/example5.png)
Input: n = 4, x = 1, y = 1 Output: [6,4,2,0] Explanation: For each distance k the pairs are: - For k == 1, the pairs are (1, 2), (2, 1), (2, 3), (3, 2), (3, 4), and (4, 3). - For k == 2, the pairs are (1, 3), (3, 1), (2, 4), and (4, 2). - For k == 3, the pairs are (1, 4), and (4, 1). - For k == 4, there are no pairs.
Constraints:
2 <= n <= 100
1 <= x, y <= n
Solutions
Solution 1: Enumeration
We can enumerate each pair of points $(i, j)$. The shortest distance from $i$ to $j$ is $min(|i - j|, |i - x| + 1 + |j - y|, |i - y| + 1 + |j - x|)$. We add $2$ to the count of this distance because both $(i, j)$ and $(j, i)$ are valid pairs of points.
The time complexity is $O(n^2)$, where $n$ is the $n$ given in the problem. Ignoring the space consumption of the answer array, the space complexity is $O(1)$.
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