# 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]], , , ]
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 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);
*/