p384

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LeetCode 384 Shuffle an Array 题解

1.题目：

Shuffle a set of numbers without duplicates.
Example:
// Init an array with set 1, 2, and 3.
int[] nums = {1,2,3};
Solution solution = new Solution(nums);
// Shuffle the array [1,2,3] and return its result. Any permutation of [1,2,3] must equally likely to be returned.
solution.shuffle();
// Resets the array back to its original configuration [1,2,3].
solution.reset();
// Returns the random shuffling of array [1,2,3].
solution.shuffle();

题意：

给你一个数组，写他的恢复原状和随机打乱（要求概率相等）功能。

2.解题思路:

见代码

3.代码

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public class Solution {

    private int[] num = null;
	private Random random = new Random();

	public Solution(int[] nums) {

		this.num = nums;
	}

	/** Resets the array to its original configuration and return it. */
	public int[] reset() {

		return num;
	}

	/** Returns a random shuffling of the array. */
	public int[] shuffle() {

		int[] n = num.clone();
		if (n.length < 2)
			return n;
		for (int i = 0; i < n.length; i++) {
			int j = random.nextInt(i+1);// 由之前的题目已经证明每个数被选中的概率相同
			// 所以他们相互被选中用来交换的概率相同，同样用数学归纳法推理。
			int c = n[i];
			n[i] = n[j];
			n[j] = c;
		}
		return n;
	}
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int[] param_1 = obj.reset();
 * int[] param_2 = obj.shuffle();
 */

4.一些总结：