3243. Shortest Distance After Road Addition Queries I

Problem

You are given an integer n and a 2D integer array queries.

There are n cities numbered from 0 to n - 1. Initially, there is a unidirectional road from city i to city i + 1 for all 0 <= i < n - 1.

queries[i] = [u_i, v_i] represents the addition of a new unidirectional road from city u_i to city v_i. After each query, you need to find the length of the shortest path from city 0 to city n - 1.

Return an array answer where for each i in the range [0, queries.length - 1], answer[i] is the length of the shortest path from city 0 to city n - 1 after processing the first i + 1 queries.

https://leetcode.cn/problems/shortest-distance-after-road-addition-queries-i/

Example 1:

Input: n = 5, queries = [[2,4],[0,2],[0,4]]
Output: [3,2,1]
Explanation:

After the addition of the road from 2 to 4, the length of the shortest path from 0 to 4 is 3.

After the addition of the road from 0 to 2, the length of the shortest path from 0 to 4 is 2.

After the addition of the road from 0 to 4, the length of the shortest path from 0 to 4 is 1.

Example 2:

Input: n = 4, queries = [[0,3],[0,2]]
Output: [1,1]
Explanation:

After the addition of the road from 0 to 3, the length of the shortest path from 0 to 3 is 1.

After the addition of the road from 0 to 2, the length of the shortest path remains 1.

Constraints:

• 3 <= n <= 500
• 1 <= queries.length <= 500
• queries[i].length == 2
• 0 <= queries[i][0] < queries[i][1] < n
• 1 < queries[i][1] - queries[i][0]
• There are no repeated roads among the queries.

Test Cases

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class Solution:
    def shortestDistanceAfterQueries(self, n: int, queries: List[List[int]]) -> List[int]:

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import pytest

from solution import Solution


@pytest.mark.parametrize('n, queries, expected', [
    (5, [[2,4],[0,2],[0,4]], [3,2,1]),
    (4, [[0,3],[0,2]], [1,1]),

    (6, [[1,4],[2,4]], [3,3],)
])
class Test:
    def test_solution(self, n, queries, expected):
        sol = Solution()
        assert sol.shortestDistanceAfterQueries(n, queries) == expected

Thoughts

显然 n - 1 到 n - 1 的最短距离是 0，记为 sd[n-1]。

如果对于 i，所有的大于 i 的 sd 都已知，可知：

$$sd[i]=1+\max_{j:\space\exists \text{ road } i \to j} sd[j]$$

时间复杂度为 O(n+q)，空间复杂度 O(n+q)（q 是 queries 的长度，即额外增加的道路数量）。

每增加一条道路 (u, v) 之后的，先更新 sd[u] 的值，如果变小了，则更新其左边所有的 sd 值。

总共时间复杂度 O(q * (n+q))。

Code

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class Solution:
    def shortestDistanceAfterQueries(self, n: int, queries: list[list[int]]) -> list[int]:
        sd = [n - i - 1 for i in range(n)]
        dests = [[i + 1] for i in range(n - 1)] # dests[i] is a list of cities which has direct road from city i.
        results = [1] * len(queries)
        for q, (u, v) in enumerate(queries):
            dests[u].append(v)
            if (d := sd[v] + 1) < sd[u]:
                sd[u] = d
                for i in range(u - 1, -1, -1):
                    sd[i] = min(sd[i], min(sd[j]+1 for j in dests[i] if j <= u))

            results[q] = sd[0]

        return results

许可协议

本文除了题目描述（Problem）部分之外，其他内容由 Calf 原创，采用 CC BY-NC-SA 4.0 许可协议，转载请注明出处。

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