package com.jack.question.random;

import java.util.Arrays;
import java.util.Random;

/**
 * @author lvh
 */
public class RandomArray {
	/*
	*  如題，定義一個100個元素的陣列arr[100];
	 * 把隨機產生的1-100之間的不重複的100個隨機數存放到此陣列中，怎樣產生隨機數才能效率高
	 */
	public static int[] getRandomNum1(int n) {
		int[] data = new int[n];
		Random random = new Random();
		int num;
		boolean bool = false;
		int arrayIndex = 0;
		while(arrayIndex < n) {
			num = random.nextInt(n);
			bool = false;

			//for(int x = 0, len = arrayIndex; x < len; x++) {
			for(int x = 0, len = arrayIndex; x < len; x++) {
				//從連結串列頭到連結串列尾查詢這個值, 時間複雜度O(N^2)
				if(data[x] == num) {
					bool = true;
					break;
				}
			}
			if(!bool) {
				data[arrayIndex] = num;
				arrayIndex++;
			}
		}
		return data;
	}
	
	public static int[] getRandomNum2(int n) {
		int[] input = new int[n];
		Random random = new Random();
		for(int x = 0 ; x < n ; x ++) {
			input[x] = x;
		}
		
		int[] out = new int[n];
		int randomIndex ;
		int arrayLen = n;
		for(int x = 0; x < n ; x++) {
			randomIndex = random.nextInt(arrayLen);
			out[x] = input[randomIndex];
			
			//刪除元素,後面的元素前移一位
			for(int y = randomIndex; y < n - 1; y++) {
				input[y] = input[y+1];
			}
			arrayLen--;
		}
		return out;
	}
	
	//時間複雜度O(n)
	public static int[] getRandomNum3(int n) {
		int[] input = new int[n];
		Random random = new Random();
		for(int x = 0 ; x < n ; x ++) {
			input[x] = x;
		}
		
		int[] out = new int[n];
		int randomIndex ;
		int arrayLen = n;
		for(int x = 0; x < n ; x++) {
			randomIndex = random.nextInt(arrayLen);
			out[x] = input[randomIndex];
			
			//有效數字前移
			if(randomIndex != arrayLen - 1) {
				input[randomIndex] = input[arrayLen - 1];
			}
			
			arrayLen--;
		}
		return out;
	}
	
	public static void main(String[] args) {
		int num = 30000;
		long begin1 = System.currentTimeMillis();
		getRandomNum1(num);
		long end1 = System.currentTimeMillis();
		System.out.println(end1 - begin1);
		
		long begin2 = System.currentTimeMillis();
		getRandomNum2(num);
		long end2 = System.currentTimeMillis();
		System.out.println(end2 - begin2);
		
		long begin3 = System.currentTimeMillis();
		getRandomNum3(num);
		long end3 = System.currentTimeMillis();
		System.out.println(end3 - begin3);
	}

}