398. Random Pick Index

Medium
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.

解題

type Solution struct {
index map[int][]int
}
​
​
func Constructor(nums []int) Solution {
m := make(map[int][]int)
for i, num := range nums {
if _, ok := m[num]; ok {
m[num] = append(m[num], i)
} else {
m[num] = []int{i}
}
}
​
return Solution{ index: m }
}
​
​
func (this *Solution) Pick(target int) int {
rand.Seed(time.Now().UnixNano())
r := rand.Intn(len(this.index[target]))
return this.index[target][r]
}
​
​
/**
* Your Solution object will be instantiated and called as such:
* obj := Constructor(nums);
* param_1 := obj.Pick(target);
*/