05 August 2008
随机shuffle array中的元素
Knuth洗牌算法，对于下标为 0 位置，从 [0, n - 1] 随机一个位置进行交换，共有 nn 种选择；下标为 11 的位置，从 [1, n - 1] 随机一个位置进行交换，共有 n - 1 种选择 ，以此类推
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class Solution {
int len;
int[] originalArr;
Random random = new Random();
public Solution(int[] nums) {
len = nums.length;
originalArr = nums;
random = new Random();
}
public int[] reset() {
return originalArr;
}
public int[] shuffle() {
// 要是不用reset只要求shuffle的话，这个数组可不要，O(1)
int[] result = originalArr.clone();
for (int i = 0; i < len; i++) {
swap(result, i, i + random.nextInt(len - i));
}
return result;
}
private void swap(int[] arr, int i, int j) {
int temp = arr[i];
arr[i] = arr[j];
arr[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();
*/
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class Solution {
private int[] nums;
public Solution(int[] nums) {
this.nums = nums;
}
/** 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() {
int[] rand = new int[nums.length];
for (int i = 0; i < nums.length; i++){
int r = (int) (Math.random() * (i+1));
rand[i] = rand[r];
rand[r] = nums[i];
}
return rand;
}
}
/**
* 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();
*/