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Question

Formatted question description: https://leetcode.ca/all/398.html

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.

Algorithm

Define two variables, counter cnt and return result res.

Traverse the entire array,

  • If the value of the array is not equal to target, skip directly;
  • If it is equal to target, the counter is increased by 1,

Then we randomly generate a number in the range of [0,cnt). If this number is 0, we assign res.

Code

  • import java.util.Random;
    
    public class Random_Pick_Index {
    
        // ref: https://leetcode.com/problems/random-pick-index/discuss/88072/Simple-Reservoir-Sampling-solution
        class Solution {
    
            int[] nums;
            Random rnd;
    
            public Solution(int[] nums) {
                this.nums = nums;
                this.rnd = new Random();
            }
    
    
            public int pick(int target) {
    
                int resultIndex = -1;
                int count = 0;
    
                for (int i = 0; i < nums.length; i++) {
                    if (nums[i] != target) {
                        continue;
                    }
    
                    count++;
    
                    if (rnd.nextInt(count) == 0) {
                        resultIndex = i;
                        // first time nextInt is always 0, so set as first index
                        // for the rest, probobility issue
                    }
                }
    
                return resultIndex;
            }
        }
    
    /**
     * Your Solution object will be instantiated and called as such:
     * Solution obj = new Solution(nums);
     * int param_1 = obj.pick(target);
     */
    }
    
    ############
    
    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);
     */
    
  • 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)
    
    ############
    
    class Solution(object):
    
      def __init__(self, nums):
        """
        
        :type nums: List[int]
        :type numsSize: int
        """
        self.nums = nums
    
      def pick(self, target):
        """
        :type target: int
        :rtype: int
        """
        count = 0
        ans = -1
        for i in range(0, len(self.nums)):
          if self.nums[i] == target:
            count += 1
            if random.randrange(0, count) == 0:
              ans = i
        return ans
    
    # Your Solution object will be instantiated and called as such:
    # obj = Solution(nums)
    # param_1 = obj.pick(target)
    
    
  • 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);
     */
    
  • 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);
     */
    

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