LeetCode 0398. Random Pick Index Solution in Java, Python, C++, JavaScript, Go & Rust | Explanation + Code

Description

Given an integer array nums 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.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the array nums.
• int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i's, then each index should have an equal probability of returning.

 

Example 1:

Input
["Solution", "pick", "pick", "pick"]
[[[1, 2, 3, 3, 3]], [3], [1], [3]]
Output
[null, 4, 0, 2]

Explanation
Solution solution = new Solution([1, 2, 3, 3, 3]);
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

 

Constraints:

• 1 <= nums.length <= 2 * 104
• -231 <= nums[i] <= 231 - 1
• target is an integer from nums.
• At most 104 calls will be made to pick.

Solutions

Solution 1

PythonJavaC++Go

class Solution: def __init__(self, nums: List[int]): self.nums = nums def pick(self, target: int) -> int: n = ans = 0 for i, v in enumerate(self.nums): if v == target: n += 1 x = random.randint(1, n) if x == n: ans = i return ans # Your Solution object will be instantiated and called as such: # obj = Solution(nums) # param_1 = obj.pick(target)(code-box)

class Solution { private int[] nums; private Random random = new Random(); public Solution(int[] nums) { this.nums = nums; } public int pick(int target) { int n = 0, ans = 0; for (int i = 0; i < nums.length; ++i) { if (nums[i] == target) { ++n; int x = 1 + random.nextInt(n); if (x == n) { ans = i; } } } return ans; } } /** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(nums); * int param_1 = obj.pick(target); */(code-box)

class Solution { public: vector<int> nums; Solution(vector<int>& nums) { this->nums = nums; } int pick(int target) { int n = 0, ans = 0; for (int i = 0; i < nums.size(); ++i) { if (nums[i] == target) { ++n; int x = 1 + rand() % n; if (n == x) ans = i; } } return ans; } }; /** * Your Solution object will be instantiated and called as such: * Solution* obj = new Solution(nums); * int param_1 = obj->pick(target); */(code-box)

type Solution struct { nums []int } func Constructor(nums []int) Solution { return Solution{nums} } func (this *Solution) Pick(target int) int { n, ans := 0, 0 for i, v := range this.nums { if v == target { n++ x := 1 + rand.Intn(n) if n == x { ans = i } } } return ans } /** * Your Solution object will be instantiated and called as such: * obj := Constructor(nums); * param_1 := obj.Pick(target); */(code-box)