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);
Thoughts
{1,2,3,3,3} with target 3, you want to select 2,3,4 with a probability of 1/3 each.
2 : It's probability of selection is 1 (1/2) (2/3) = 1/3
3 : It's probability of selection is (1/2) * (2/3) = 1/3
4 : It's probability of selection is just 1/3
Code
/* * @lc app=leetcode id=398 lang=cpp * * [398] Random Pick Index * * https://leetcode.com/problems/random-pick-index/description/ * * algorithms * Medium (50.92%) * Likes: 293 * Dislikes: 495 * Total Accepted: 61.5K * Total Submissions: 119.8K * Testcase Example: '["Solution","pick"]\n[[[1,2,3,3,3]],[3]]' * * 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); * * */classSolution {public: vector<int> _nums;Solution(vector<int>& nums) { _nums = nums; }intpick(int target) {int k =1, count =0; vector<int>res(k);for (int i =0; i <_nums.size(); ++i) {if (_nums[i] != target) continue;if (count < k) {res[count++] = i; } else {constint r =rand() % (++count);if (r < k) res[r] = i; } }returnres[k -1]; }};/** * Your Solution object will be instantiated and called as such: * Solution* obj = new Solution(nums); * int param_1 = obj->pick(target); */
classSolution {int[] nums;Random rand;publicSolution(int[] nums) {this.nums= nums; rand =newRandom(); }publicintpick(int target) {int count =0;int k =1;int[] res =newint[k];for (int i =0; i <nums.length; i++) {if (nums[i] == target) {if (count < k) { res[count++] = i; } else {int r =rand.nextInt(++count);if (r < k) { res[r] = i; } } } }return res[0]; }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */