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  • LeetCode 384. Shuffle an Array

    原题链接在这里:https://leetcode.com/problems/shuffle-an-array/

    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();

    题解:

    Shuffle时每个数字在小于它的方位内 swap 一次.

    j = random.nextInt(i+1). j is picked randomly from [0,i].

    有两种可能

    1. j == i, 那么nums[i]就没有换位置. 概率是1/(1+i).

    2. j != i, 那么nums[i]就换位置了, 另外[0, i-1] 之间 i choices, 任选一个数与其对调. 概率是 (1-1/(1+i)) * (1/i) = 1/(1+i).

    以此类推后就表明,结果中任何一个数字都有相等的概率在任意个位置上. 就是any permutation has same 概率.

    Time Complexity: reset, O(1). shuffle, O(nums.length).

    Space: O(nums.length).

    AC Java:

     1 class Solution {
     2     int [] nums;
     3     Random random;
     4     
     5     public Solution(int[] nums) {
     6         this.nums = nums;
     7         random = new Random();
     8     }
     9     
    10     /** Resets the array to its original configuration and return it. */
    11     public int[] reset() {
    12         return this.nums;
    13     }
    14     
    15     /** Returns a random shuffling of the array. */
    16     public int[] shuffle() {
    17         int [] arr = nums.clone();
    18         for(int i = 0; i<nums.length; i++){
    19             int j = random.nextInt(i+1);
    20             swap(arr, i, j);
    21         }
    22         
    23         return arr;
    24     }
    25     
    26     private void swap(int [] nums, int i, int j){
    27         int temp = nums[i];
    28         nums[i] = nums[j];
    29         nums[j] = temp;
    30     }
    31 }
    32 
    33 /**
    34  * Your Solution object will be instantiated and called as such:
    35  * Solution obj = new Solution(nums);
    36  * int[] param_1 = obj.reset();
    37  * int[] param_2 = obj.shuffle();
    38  */
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  • 原文地址:https://www.cnblogs.com/Dylan-Java-NYC/p/7929891.html
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