algorithm-class/大厂刷题班/class04/Code01_QueryHobby.java

package class04;

import java.util.ArrayList;
import java.util.HashMap;

public class Code01_QueryHobby {

	/*
	 * 今日头条原题
	 * 
	 * 数组为{3, 2, 2, 3, 1}，查询为(0, 3, 2)。意思是在数组里下标0~3这个范围上，有几个2？返回2。
	 * 假设给你一个数组arr，对这个数组的查询非常频繁，请返回所有查询的结果
	 * 
	 */

	public static class QueryBox1 {
		private int[] arr;

		public QueryBox1(int[] array) {
			arr = new int[array.length];
			for (int i = 0; i < arr.length; i++) {
				arr[i] = array[i];
			}
		}

		public int query(int L, int R, int v) {
			int ans = 0;
			for (; L <= R; L++) {
				if (arr[L] == v) {
					ans++;
				}
			}
			return ans;
		}
	}

	public static class QueryBox2 {
		private HashMap<Integer, ArrayList<Integer>> map;

		public QueryBox2(int[] arr) {
			map = new HashMap<>();
			for (int i = 0; i < arr.length; i++) {
				if (!map.containsKey(arr[i])) {
					map.put(arr[i], new ArrayList<>());
				}
				map.get(arr[i]).add(i);
			}
		}

		public int query(int L, int R, int value) {
			if (!map.containsKey(value)) {
				return 0;
			}
			ArrayList<Integer> indexArr = map.get(value);
			// 查询 < L 的下标有几个
			int a = countLess(indexArr, L);
			// 查询 < R+1 的下标有几个
			int b = countLess(indexArr, R + 1);
			return b - a;
		}

		// 在有序数组arr中，用二分的方法数出<limit的数有几个
		// 也就是用二分法，找到<limit的数中最右的位置
		private int countLess(ArrayList<Integer> arr, int limit) {
			int L = 0;
			int R = arr.size() - 1;
			int mostRight = -1;
			while (L <= R) {
				int mid = L + ((R - L) >> 1);
				if (arr.get(mid) < limit) {
					mostRight = mid;
					L = mid + 1;
				} else {
					R = mid - 1;
				}
			}
			return mostRight + 1;
		}

	}

	public static int[] generateRandomArray(int len, int value) {
		int[] ans = new int[(int) (Math.random() * len) + 1];
		for (int i = 0; i < ans.length; i++) {
			ans[i] = (int) (Math.random() * value) + 1;
		}
		return ans;
	}

	public static void main(String[] args) {
		int len = 300;
		int value = 20;
		int testTimes = 1000;
		int queryTimes = 1000;
		System.out.println("test begin");
		for (int i = 0; i < testTimes; i++) {
			int[] arr = generateRandomArray(len, value);
			int N = arr.length;
			QueryBox1 box1 = new QueryBox1(arr);
			QueryBox2 box2 = new QueryBox2(arr);
			for (int j = 0; j < queryTimes; j++) {
				int a = (int) (Math.random() * N);
				int b = (int) (Math.random() * N);
				int L = Math.min(a, b);
				int R = Math.max(a, b);
				int v = (int) (Math.random() * value) + 1;
				if (box1.query(L, R, v) != box2.query(L, R, v)) {
					System.out.println("Oops!");
				}
			}
		}
		System.out.println("test end");
	}

}