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);
 */
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