題目要求

P5143題目鏈接

分析

一個排序求解的題，C++很容易AC，但Java黨需要做一些性能優化，下面做一下分析和總結。

第一版代碼（80分）：

import java.util.Arrays; import java.util.Comparator; import java.util.Scanner;public class Main {private static class Hill {int x;int y;int z;public Hill(int x, int y, int z) {this.x = x;this.y = y;this.z = z;}}public static void main(String[] args) {Scanner scanner = new Scanner(System.in);int num = scanner.nextInt();Hill[] hills = new Hill[num];for (int i = 0; i < num; i++) {hills[i] = new Hill(scanner.nextInt(), scanner.nextInt(), scanner.nextInt());}Arrays.sort(hills, Comparator.comparing(e -> e.z));scanner.close();int x = hills[0].x, y = hills[0].y, z = hills[0].z;double sum = 0;for (int i = 1; i < num; i++) {sum += Math.sqrt(Math.pow(hills[i].x-x, 2) + Math.pow(hills[i].y-y, 2) + Math.pow(hills[i].z-z, 2));x = hills[i].x;y = hills[i].y;z = hills[i].z;}System.out.printf("%.3f", sum);}}

第二版代碼（80分）：

將Scanner換成了BufferedReader，個別點有所優化，整體沒有突破性能瓶頸。

import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.Arrays; import java.util.Comparator;public class Main {private static class Hill {int x;int y;int z;public Hill(int x, int y, int z) {this.x = x;this.y = y;this.z = z;}}public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));int num = Integer.parseInt(reader.readLine());Hill[] hills = new Hill[num];for (int i = 0; i < num; i++) {String[] temp = reader.readLine().split("\\s+");hills[i] = new Hill(Integer.parseInt(temp[0]), Integer.parseInt(temp[1]), Integer.parseInt(temp[2]));}Arrays.sort(hills, Comparator.comparing(e -> e.z));reader.close();int x = hills[0].x, y = hills[0].y, z = hills[0].z;double sum = 0;for (int i = 1; i < num; i++) {sum += Math.sqrt(Math.pow(hills[i].x-x, 2) + Math.pow(hills[i].y-y, 2) + Math.pow(hills[i].z-z, 2));x = hills[i].x;y = hills[i].y;z = hills[i].z;}System.out.printf("%.3f", sum);}}

第三版代碼（80分）：

使用short替換int，賭測試數據不會爆short，結果真的爆了，導致了RE。

import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.Arrays; import java.util.Comparator;public class Main {private static class Hill {short x;short y;short z;Hill(short x, short y, short z) {this.x = x;this.y = y;this.z = z;}}public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));int num = Integer.parseInt(reader.readLine());Hill[] hills = new Hill[num];for (int i = 0; i < num; i++) {String[] temp = reader.readLine().split("\\s+");hills[i] = new Hill(Short.parseShort(temp[0]), Short.parseShort(temp[1]), Short.parseShort(temp[2]));}Arrays.sort(hills, Comparator.comparing(e -> e.z));reader.close();short x = hills[0].x, y = hills[0].y, z = hills[0].z;double sum = 0;for (int i = 1; i < num; i++) {sum += Math.sqrt(Math.pow(hills[i].x-x, 2) + Math.pow(hills[i].y-y, 2) + Math.pow(hills[i].z-z, 2));x = hills[i].x;y = hills[i].y;z = hills[i].z;}System.out.printf("%.3f", sum);}}

第四版代碼（90分）：

換回int的情況下，懷疑Math.pow()的效率問題，改成了直接相乘，解決了最后一個點，但倒數第二個點反而慢了一些。

import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.Arrays; import java.util.Comparator;public class Main {private static class Hill {int x;int y;int z;Hill(int x, int y, int z) {this.x = x;this.y = y;this.z = z;}}public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));int num = Integer.parseInt(reader.readLine());Hill[] hills = new Hill[num];for (int i = 0; i < num; i++) {String[] temp = reader.readLine().split("\\s+");hills[i] = new Hill(Integer.parseInt(temp[0]), Integer.parseInt(temp[1]), Integer.parseInt(temp[2]));}Arrays.sort(hills, Comparator.comparing(e -> e.z));reader.close();int x = hills[0].x, y = hills[0].y, z = hills[0].z;double sum = 0;for (Hill hill : hills) {sum += Math.sqrt((hill.x-x)*(hill.x-x) + (hill.y-y)*(hill.y-y) + (hill.z-z)*(hill.z-z));x = hill.x;y = hill.y;z = hill.z;}System.out.printf("%.3f", sum);}}

第五版代碼（90分）：

嘗試推翻sort()的約束，換成TreeSet來排序，但整體瓶頸點有所優化，倒數第二個點突破了，但最后一個點還差一點點。

import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.*;public class Main {private static class Hill {int x;int y;int z;Hill(int x, int y, int z) {this.x = x;this.y = y;this.z = z;}}public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));int num = Integer.parseInt(reader.readLine());Set<Hill> set = new TreeSet<>(Comparator.comparing(e -> e.z));for (int i = 0; i < num; i++) {String[] temp = reader.readLine().split("\\s+");set.add(new Hill(Integer.parseInt(temp[0]), Integer.parseInt(temp[1]), Integer.parseInt(temp[2])));}reader.close();Iterator<Hill> iterator = set.iterator();Hill hill0 = iterator.next();int x = hill0.x, y = hill0.y, z = hill0.z;double sum = 0;while (iterator.hasNext()) {Hill hill = iterator.next();sum += Math.sqrt((hill.x-x)*(hill.x-x) + (hill.y-y)*(hill.y-y) + (hill.z-z)*(hill.z-z));x = hill.x;y = hill.y;z = hill.z;}System.out.printf("%.3f", sum);}}

第六版代碼（100分）：

突然想到，是不是\\s+的匹配效率不如 ，就決定換一下，最終通過。沒想到性能優化了這么多。

AC代碼（Java語言描述）

import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.*;public class Main {private static class Hill {int x;int y;int z;Hill(int x, int y, int z) {this.x = x;this.y = y;this.z = z;}}public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));int num = Integer.parseInt(reader.readLine());Set<Hill> set = new TreeSet<>(Comparator.comparing(e -> e.z));for (int i = 0; i < num; i++) {String[] temp = reader.readLine().split(" ");set.add(new Hill(Integer.parseInt(temp[0]), Integer.parseInt(temp[1]), Integer.parseInt(temp[2])));}reader.close();Iterator<Hill> iterator = set.iterator();Hill hill0 = iterator.next();int x = hill0.x, y = hill0.y, z = hill0.z;double sum = 0;while (iterator.hasNext()) {Hill hill = iterator.next();sum += Math.sqrt((hill.x-x)*(hill.x-x) + (hill.y-y)*(hill.y-y) + (hill.z-z)*(hill.z-z));x = hill.x;y = hill.y;z = hill.z;}System.out.printf("%.3f", sum);}}

總結

由于這次只差一點點，所以并沒有放棄Java，而是自己去琢磨，并總結出了如下經驗：

使用\\s+的匹配效率不如 ，在保證可以使用空格的情況下可以直接用。

直接乘法的效率可能高于使用一次Math.pow(, 2)。

sort()未必是最優策略，有時TreeSet也是一種好手段。

迭代器或者增強for循環的效率是優于普通for循環的。

BufferedReader比Scanner固然是優秀很多的，但未必能完全解決問題。有的優化就在一些小細節上。

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