PAT 甲级 1143 Lowest Common Ancestor-CFANZ编程社区

1143 Lowest Common Ancestor （30 point(s)）

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.

经验总结：

emmmmm 首先要明确LCA到底是什么，即两个结点a,b的最低公共祖先，所以从前往后遍历先序序列，找到第一个x，使得x>=a&&x<=b||x<=a&&x>=b即可，这里一定要注意字符串不要写错了，我就是字符串里多加了一个点，找了半个小时才找出来错误在哪T-T

AC代码

#include <cstdio>
#include <vector>
#include <unordered_map>
using namespace std;
const int maxn=10010;
unordered_map<int,bool> mp;
int n,m,a,b,pre[maxn];
int main()
{
  scanf("%d%d",&m,&n);
  for(int i=0;i<n;++i)
  {
    scanf("%d",&pre[i]);
    mp[pre[i]]=true;
  }
  int x;
  for(int i=0;i<m;++i)
  {
    scanf("%d%d",&a,&b);
    if(mp[a]==false&&mp[b]==false)
      printf("ERROR: %d and %d are not found.\n",a,b);
    else if(mp[a]==false||mp[b]==false)
      printf("ERROR: %d is not found.\n",mp[a]==false?a:b);
    else
    {
      for(int j=0;j<n;++j)
      {
        x=pre[j];
        if(x>=a&&x<=b||x>=b&&x<=a)
          break;
      }
      if(x==a||x==b)
        printf("%d is an ancestor of %d.\n",x,x==a?b:a);
      else
        printf("LCA of %d and %d is %d.\n",a,b,x);
    }
  }
  return 0;
}