{ "data": { "question": { "questionId": "3906", "questionFrontendId": "3590", "categoryTitle": "Algorithms", "boundTopicId": 3705403, "title": "Kth Smallest Path XOR Sum", "titleSlug": "kth-smallest-path-xor-sum", "content": "
You are given an undirected tree rooted at node 0 with n
nodes numbered from 0 to n - 1
. Each node i
has an integer value vals[i]
, and its parent is given by par[i]
.
The path XOR sum from the root to a node u
is defined as the bitwise XOR of all vals[i]
for nodes i
on the path from the root node to node u
, inclusive.
You are given a 2D integer array queries
, where queries[j] = [uj, kj]
. For each query, find the kjth
smallest distinct path XOR sum among all nodes in the subtree rooted at uj
. If there are fewer than kj
distinct path XOR sums in that subtree, the answer is -1.
Return an integer array where the jth
element is the answer to the jth
query.
In a rooted tree, the subtree of a node v
includes v
and all nodes whose path to the root passes through v
, that is, v
and its descendants.
\n
Example 1:
\n\nInput: par = [-1,0,0], vals = [1,1,1], queries = [[0,1],[0,2],[0,3]]
\n\nOutput: [0,1,-1]
\n\nExplanation:
\n\nPath XORs:
\n\n1
1 XOR 1 = 0
1 XOR 1 = 0
Subtree of 0: Subtree rooted at node 0 includes nodes [0, 1, 2]
with Path XORs = [1, 0, 0]
. The distinct XORs are [0, 1]
.
Queries:
\n\nqueries[0] = [0, 1]
: The 1st smallest distinct path XOR in the subtree of node 0 is 0.queries[1] = [0, 2]
: The 2nd smallest distinct path XOR in the subtree of node 0 is 1.queries[2] = [0, 3]
: Since there are only two distinct path XORs in this subtree, the answer is -1.Output: [0, 1, -1]
Example 2:
\n\nInput: par = [-1,0,1], vals = [5,2,7], queries = [[0,1],[1,2],[1,3],[2,1]]
\n\nOutput: [0,7,-1,0]
\n\nExplanation:
\n\nPath XORs:
\n\n5
5 XOR 2 = 7
5 XOR 2 XOR 7 = 0
Subtrees and Distinct Path XORs:
\n\n[0, 1, 2]
with Path XORs = [5, 7, 0]
. The distinct XORs are [0, 5, 7]
.[1, 2]
with Path XORs = [7, 0]
. The distinct XORs are [0, 7]
.[2]
with Path XOR = [0]
. The distinct XORs are [0]
.Queries:
\n\nqueries[0] = [0, 1]
: The 1st smallest distinct path XOR in the subtree of node 0 is 0.queries[1] = [1, 2]
: The 2nd smallest distinct path XOR in the subtree of node 1 is 7.queries[2] = [1, 3]
: Since there are only two distinct path XORs, the answer is -1.queries[3] = [2, 1]
: The 1st smallest distinct path XOR in the subtree of node 2 is 0.Output: [0, 7, -1, 0]
\n
Constraints:
\n\n1 <= n == vals.length <= 5 * 104
0 <= vals[i] <= 105
par.length == n
par[0] == -1
0 <= par[i] < n
for i
in [1, n - 1]
1 <= queries.length <= 5 * 104
queries[j] == [uj, kj]
0 <= uj < n
1 <= kj <= n
par
represents a valid tree.给定一棵以节点 0 为根的无向树,带有 n
个节点,按 0 到 n - 1
编号。每个节点 i
有一个整数值 vals[i]
,并且它的父节点通过 par[i]
给出。
从根节点 0 到节点 u
的 路径异或和 定义为从根节点到节点 u
的路径上所有节点 i
的 vals[i]
的按位异或,包括节点 u
。
给定一个 2 维整数数组 queries
,其中 queries[j] = [uj, kj]
。对于每个查询,找到以 uj
为根的子树的所有节点中,第 kj
小 的 不同 路径异或和。如果子树中 不同 的异或路径和少于 kj
,答案为 -1。
返回一个整数数组,其中第 j
个元素是第 j
个查询的答案。
在有根树中,节点 v
的子树包括 v
以及所有经过 v
到达根节点路径上的节点,即 v
及其后代节点。
\n\n
示例 1:
\n\n输入:par = [-1,0,0], vals = [1,1,1], queries = [[0,1],[0,2],[0,3]]
\n\n输出:[0,1,-1]
\n\n解释:
\n\n路径异或值:
\n\n1
1 XOR 1 = 0
1 XOR 1 = 0
0 的子树:以节点 0 为根的子树包括节点 [0, 1, 2]
,路径异或值为 [1, 0, 0]
。不同的异或值为 [0, 1]
。
查询:
\n\nqueries[0] = [0, 1]
:节点 0 的子树中第 1 小的不同路径异或值为 0。queries[1] = [0, 2]
:节点 0 的子树中第 2 小的不同路径异或值为 1。queries[2] = [0, 3]
:由于子树中只有两个不同路径异或值,答案为 -1。输出:[0, 1, -1]
示例 2:
\n\n输入:par = [-1,0,1], vals = [5,2,7], queries = [[0,1],[1,2],[1,3],[2,1]]
\n\n输出:[0,7,-1,0]
\n\n解释:
\n\n路径异或值:
\n\n5
5 XOR 2 = 7
5 XOR 2 XOR 7 = 0
子树与不同路径异或值:
\n\n[0, 1, 2]
,路径异或值为 [5, 7, 0]
。不同的异或值为 [0, 5, 7]
。[1, 2]
,路径异或值为 [7, 0]
。不同的异或值为 [0, 7]
。[2]
,路径异或值为 [0]
。不同的异或值为 [0]
。查询:
\n\nqueries[0] = [0, 1]
:节点 0 的子树中,第 1 小的不同路径异或值为 0。queries[1] = [1, 2]
:节点 1 的子树中,第 2 小的不同路径异或值为 7。queries[2] = [1, 3]
:由于子树中只有两个不同路径异或值,答案为 -1。queries[3] = [2, 1]
:节点 2 的子树中,第 1 小的不同路径异或值为 0。输出:[0, 7, -1, 0]
\n\n
提示:
\n\n1 <= n == vals.length <= 5 * 104
0 <= vals[i] <= 105
par.length == n
par[0] == -1
[1, n - 1]
中的 i
,0 <= par[i] < n
1 <= queries.length <= 5 * 104
queries[j] == [uj, kj]
0 <= uj < n
1 <= kj <= n
par
表示一棵合法的树。u
, maintain the set of XOR values along the path from the root to u
.",
"Use DSU on tree (small‑to‑large merging) during DFS to efficiently merge each child's set into its parent's set.",
"Store all XOR values in an ordered_set
(in Python you can use the sortedcontainers
module's SortedList
) so you can quickly find the k
th smallest XOR in any subtree.",
"At node u
, process each query [u, k]
by calling find_by_order(k − 1)
(C++ PBDS) or indexing sorted_list[k-1]
(Python SortedList
)."
],
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