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题目描述：
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();
解题思路：
每次往后读取数组的时候，当读到第i个的时候以1/i的概率随机替换1～i中的任何一个数，这样保证最后每个数字出现在每个位置上的概率都是相等的。
证明：
设x元素在第m次的时候出现在位置i的概率是1/m,那么在第m+1次的时候，x仍然待在位置i的概率是 1/m * m/(m+1) = 1/(m+1)
代码描述：
public class Solution {
private int[] nums = null;
private Random random = null;
public Solution(int[] nums) {
this.nums = nums;
random = new Random();
}
/** Resets the array to its original configuration and return it. */
public int[] reset() {
return nums;
}
/** Returns a random shuffling of the array. */
public int[] shuffle() {
if(nums == null) return null;
int[] a = (int[])nums.clone();
for(int i = 1; i < nums.length; i++){
int j = random.nextInt(i + 1);
swap(a, i, j);
}
return a;
}
private void swap(int[] a, int i, int j){
int temp = a[i];
a[i] = a[j];
a[j] = temp;
}
}
/**
* 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();
*/
Java [Leetcode 384]Shuffle an Array
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原文地址：http://www.cnblogs.com/zihaowang/p/5801312.html
