find random maximum

最新推荐文章于 2024-10-22 08:15:00 发布

sjphiChina

最新推荐文章于 2024-10-22 08:15:00 发布

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本文介绍了一种从整数数组中随机选择最大值索引的方法，并提供了三种不同的实现方式：使用列表存储最大值索引、修改原数组来存储最大值索引以及使用蓄水池抽样算法进行单次遍历。

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就是一个int array 里面会有一到多个maximum，返回一个随机maximum的index

// 2 pass, o(n) time, o(n) space
	public int index(int[] nums) {
		int max = Integer.MIN_VALUE;
		List<Integer> list = new ArrayList<>();
		for (int i = 0; i < nums.length; i++) {
			max = Math.max(max, nums[i]);
		}

		for (int i = 0; i < nums.length; i++) {
			if (max == nums[i]) {
				list.add(i);
			}
		}
		return list.get(new Random().nextInt(list.size()));
	}

	// wrong
	// two pass, n time, 1 space
	public int index0(int[] nums) {
		int max = Integer.MIN_VALUE;
		for (int i = 0; i < nums.length; i++) {
			max = Math.max(max, nums[i]);
		}
		int j = 0;
		for (int i = 0; i < nums.length; i++) {
			if (max == nums[i]) {
				nums[j] = i;
				j++;
			}
		}
		return nums[new Random().nextInt(j)];
	}

	// one pass
	public int find(int[] A) {
		Random rand = new Random();
		int[] reservior = new int[1];
		int maxValue = Integer.MIN_VALUE;
		int numOfMaxValue = 0;
		for (int i = 0; i < A.length; i++) {
			if (A[i] == maxValue) {
				numOfMaxValue++;
				int randNum = rand.nextInt(numOfMaxValue);
				if (randNum < 1) {
					reservior[0] = i;
				}
			}

			if (A[i] > maxValue) {
				maxValue = A[i];
				numOfMaxValue = 1;
				reservior[0] = i;
			}
		}
		return reservior[0];
	}

	public static void main(String[] args) {
		FindIndexOfMaxValue example = new FindIndexOfMaxValue();
		int count4 = 0, count6 = 0;
		int[] A = { 1, 2, 3, 4, 5, 3, 5 };
		for (int i = 0; i < 10000; i++) {
			int res = example.find(A);
			if (res == 4) {
				count4++;
			}
			if (res == 6) {
				count6++;
			}
		}

		System.out.println("6 appears " + count6 + " times");
		System.out.println("4 appears " + count4 + " times");

		count4 = 0;
		count6 = 0;
		int[] B = { 1, 2, 3, 4, 5, 3, 5 };
		for (int i = 0; i < 10000; i++) {
			int res = example.index(B);
			if (res == 4) {
				count4++;
			}
			if (res == 6) {
				count6++;
			}
		}
		System.out.println("6 appears " + count6 + " times");
		System.out.println("4 appears " + count4 + " times");

		count4 = 0;
		count6 = 0;
		int[] C = { 1, 2, 3, 4, 5, 3, 5 };
		for (int i = 0; i < 10000; i++) {
			int res = example.index0(C);
			if (res == 4) {
				count4++;
			}
			if (res == 6) {
				count6++;
			}
		}
		System.out.println("6 appears " + count6 + " times");
		System.out.println("4 appears " + count4 + " times");
	}