[PAT] 1143 Lowest Common Ancestor （30 分）Java

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.

package pattest;

import java.util.Scanner;
import java.util.List;
import java.util.ArrayList;

/**
 * @Auther: Xingzheng Wang
 * @Date: 2019/2/28 22:14
 * @Description: pattest
 * @Version: 1.0
 */
public class PAT1143 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        List<Integer> list = new ArrayList<>();
        int m = sc.nextInt();
        int n = sc.nextInt();
        int u, v, a = -1;
        int[] preOrder = new int[n];
        for (int i = 0; i < n; i++) {
            preOrder[i] = sc.nextInt();
            list.add(preOrder[i]);
        }
        for (int i = 0; i < m; i++) {
            u = sc.nextInt();
            v = sc.nextInt();
            for (int j = 0; j < n; j++) {
                a = j;
                if ((preOrder[j] >= u && preOrder[j] <= v) || (preOrder[j] <= u && preOrder[j] >= v)) {
                    break;
                }
            }
            if (list.contains(u) == false && list.contains(v) == false) {
                System.out.printf("ERROR: %d and %d is not found.\n", u, v);
            } else if (list.contains(u) == false || list.contains(v) == false) {
                System.out.printf("ERROR: %d is not found.\n", list.contains(u) == false ? u : v);
            } else if (preOrder[a] == u || preOrder[a] == v) {
                System.out.printf("%d is an ancestor of %d.\n", preOrder[a], preOrder[a] == u ? v : u);
            } else {
                System.out.printf("LCA of %d and %d is %d.\n", u, v, preOrder[a]);
            }
        }
    }
}