PAT 甲级 1143 Lowest Common Ancestor
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https://pintia.cn/problem-sets/994805342720868352/problems/994805343727501312
The lowest common ancestor (LCA) of two nodes U and V in a tree is the deepest node that has both U and V as descendants.
A binary search tree (BST) is recursively defined as a binary tree which has the following properties:
- The left subtree of a node contains only nodes with keys less than the node‘s key.
- The right subtree of a node contains only nodes with keys greater than or equal to the node‘s key.
- Both the left and right subtrees must also be binary search trees.
Given any two nodes in a BST, you are supposed to find their LCA.
Input Specification:
Each input file contains one test case. For each case, the first line gives two positive integers: M (≤ 1,000), the number of pairs of nodes to be tested; and N (≤10,000), the number of keys in the BST, respectively. In the second line, N distinct integers are given as the preorder traversal sequence of the BST. Then M lines follow, each contains a pair of integer keys U and V. All the keys are in the range of int.
Output Specification:
For each given pair of U and V, print in a line LCA of U and V is A.
if the LCA is found and A
is the key. But if A
is one of U and V, print X is an ancestor of Y.
where X
is A
and Y
is the other node. If U or V is not found in the BST, print in a line ERROR: U is not found.
or ERROR: V is not found.
or ERROR: U and V are not found.
.
Sample Input:
6 8
6 3 1 2 5 4 8 7
2 5
8 7
1 9
12 -3
0 8
99 99
Sample Output:
LCA of 2 and 5 is 3.
8 is an ancestor of 7.
ERROR: 9 is not found.
ERROR: 12 and -3 are not found.
ERROR: 0 is not found.
ERROR: 99 and 99 are not found.
代码:
#include <bits/stdc++.h> using namespace std; int N, M; vector<int> pre; map<int, int> mp; int main() { scanf("%d%d", &M, &N); pre.resize(N); for(int i = 0; i < N; i ++) { scanf("%d", &pre[i]); mp[pre[i]] = 1; } while(M --) { int a, b; scanf("%d%d", &a, &b); if(!mp[a] && !mp[b]) printf("ERROR: %d and %d are not found. ", a, b); else if(!mp[a] || !mp[b]) printf("ERROR: %d is not found. ", mp[a] ? b : a); else { int root; for(int i = 0; i < N; i ++) { root = pre[i]; if((root >= a && root <= b) || (root <= a && root >= b)) break; } if(root == a) printf("%d is an ancestor of %d. ", a, b); else if(root == b) printf("%d is an ancestor of %d. ", b, a); else printf("LCA of %d and %d is %d. ", a, b, root); } } return 0; }
用 mp 记下是否出现过 然后只要从头找满足值在 a b 之间的就是根了
FHFHFH 过年之前最后一个工作日
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