Problem:

https://leetcode.com/problems/random-pick-index/description/

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:

int[] nums = new int[] {1,2,3,3,3};

Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

solution.pick(3);

// pick(1) should return 0. Since in the array only nums[0] is equal to 1.

solution.pick(1);

Solution: Reservoir sampling

https://en.wikipedia.org/wiki/Reservoir_sampling

Time complexity: O(query * n)

Space complexity: O(1)

C++

// Author: Huahua

// Running time: 92 ms

class Solution {

public:

Solution(vector<int> nums) {

nums_ = std::move(nums);

}

int pick(int target) {

int n = 0;

int index = -1;

for (int i = 0; i < nums_.size(); ++i) {

if (nums_[i] != target) continue;

++n;

if (rand() % n == 0) index = i;

}

return index;

}

private:

vector<int> nums_;

};

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();

C++

// Author: Huahua

// Running time: 249 ms

class Solution {

public:

Solution(vector<int> nums) {

nums_ = std::move(nums);

}

/** Resets the array to its original configuration and return it. */

vector<int> reset() {

return nums_;

}

/** Returns a random shuffling of the array. */

vector<int> shuffle() {

vector<int> output(nums_);

const int n = nums_.size();

for (int i = 0; i < n - 1; ++i) {

int j = rand() % (n - i) + i;

std::swap(output[i], output[j]);

}

return output;

}

private:

vector<int> nums_;

};

Posts tagged as “randomization”

花花酱 LeetCode 398. Random Pick Index

Problem:

Example:

Solution: Reservoir sampling

花花酱 LeetCode 384. Shuffle an Array