Question

398. Random Pick Index

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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

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 {
  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 {
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);
 */