Question

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

 398	Random Pick Index

 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);

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

Java

  • 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);
     */
    }
    
  • Todo
    
  • 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)
    
    

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