{ "data": { "question": { "questionId": "3112", "questionFrontendId": "2867", "categoryTitle": "Algorithms", "boundTopicId": 2453611, "title": "Count Valid Paths in a Tree", "titleSlug": "count-valid-paths-in-a-tree", "content": "
There is an undirected tree with n
nodes labeled from 1
to n
. You are given the integer n
and a 2D integer array edges
of length n - 1
, where edges[i] = [ui, vi]
indicates that there is an edge between nodes ui
and vi
in the tree.
Return the number of valid paths in the tree.
\n\nA path (a, b)
is valid if there exists exactly one prime number among the node labels in the path from a
to b
.
Note that:
\n\n(a, b)
is a sequence of distinct nodes starting with node a
and ending with node b
such that every two adjacent nodes in the sequence share an edge in the tree.(a, b)
and path (b, a)
are considered the same and counted only once.\n
Example 1:
\n\n\nInput: n = 5, edges = [[1,2],[1,3],[2,4],[2,5]]\nOutput: 4\nExplanation: The pairs with exactly one prime number on the path between them are: \n- (1, 2) since the path from 1 to 2 contains prime number 2. \n- (1, 3) since the path from 1 to 3 contains prime number 3.\n- (1, 4) since the path from 1 to 4 contains prime number 2.\n- (2, 4) since the path from 2 to 4 contains prime number 2.\nIt can be shown that there are only 4 valid paths.\n\n\n
Example 2:
\n\n\nInput: n = 6, edges = [[1,2],[1,3],[2,4],[3,5],[3,6]]\nOutput: 6\nExplanation: The pairs with exactly one prime number on the path between them are: \n- (1, 2) since the path from 1 to 2 contains prime number 2.\n- (1, 3) since the path from 1 to 3 contains prime number 3.\n- (1, 4) since the path from 1 to 4 contains prime number 2.\n- (1, 6) since the path from 1 to 6 contains prime number 3.\n- (2, 4) since the path from 2 to 4 contains prime number 2.\n- (3, 6) since the path from 3 to 6 contains prime number 3.\nIt can be shown that there are only 6 valid paths.\n\n\n
\n
Constraints:
\n\n1 <= n <= 105
edges.length == n - 1
edges[i].length == 2
1 <= ui, vi <= n
edges
represent a valid tree.给你一棵 n
个节点的无向树,节点编号为 1
到 n
。给你一个整数 n
和一个长度为 n - 1
的二维整数数组 edges
,其中 edges[i] = [ui, vi]
表示节点 ui
和 vi
在树中有一条边。
请你返回树中的 合法路径数目 。
\n\n如果在节点 a
到节点 b
之间 恰好有一个 节点的编号是质数,那么我们称路径 (a, b)
是 合法的 。
注意:
\n\n(a, b)
指的是一条从节点 a
开始到节点 b
结束的一个节点序列,序列中的节点 互不相同 ,且相邻节点之间在树上有一条边。(a, b)
和路径 (b, a)
视为 同一条 路径,且只计入答案 一次 。\n\n
示例 1:
\n\n\n\n\n输入:n = 5, edges = [[1,2],[1,3],[2,4],[2,5]]\n输出:4\n解释:恰好有一个质数编号的节点路径有:\n- (1, 2) 因为路径 1 到 2 只包含一个质数 2 。\n- (1, 3) 因为路径 1 到 3 只包含一个质数 3 。\n- (1, 4) 因为路径 1 到 4 只包含一个质数 2 。\n- (2, 4) 因为路径 2 到 4 只包含一个质数 2 。\n只有 4 条合法路径。\n\n\n
示例 2:
\n\n\n\n\n输入:n = 6, edges = [[1,2],[1,3],[2,4],[3,5],[3,6]]\n输出:6\n解释:恰好有一个质数编号的节点路径有:\n- (1, 2) 因为路径 1 到 2 只包含一个质数 2 。\n- (1, 3) 因为路径 1 到 3 只包含一个质数 3 。\n- (1, 4) 因为路径 1 到 4 只包含一个质数 2 。\n- (1, 6) 因为路径 1 到 6 只包含一个质数 3 。\n- (2, 4) 因为路径 2 到 4 只包含一个质数 2 。\n- (3, 6) 因为路径 3 到 6 只包含一个质数 3 。\n只有 6 条合法路径。\n\n\n
\n\n
提示:
\n\n1 <= n <= 105
edges.length == n - 1
edges[i].length == 2
1 <= ui, vi <= n
edges
形成一棵合法的树。[1, n]
.****",
"Root the tree at any node.",
"Let dp[i][0] = the number of vertical paths starting from i containing no prime nodes
, and dp[i][1] = the number of vertical paths starting from i containing one prime node
.",
"If i
is not prime, dp[i][0] = sum(dp[child][0]) + 1
, and dp[i][1] = sum(dp[child][1])
for each child
of i
in the rooted tree.",
"If i
is prime, dp[i][0] = 0
, and dp[i][1] = sum(dp[child][0]) + 1
for each child
of i
in the rooted tree.",
"For each node i
, and using the computed dp
matrix, count the number of unordered pairs (a,b)
such that lca(a,b) = i
, and there exists exactly one prime number on the path from a
to b
."
],
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"status": null,
"sampleTestCase": "5\n[[1,2],[1,3],[2,4],[2,5]]",
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