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### 题目描述

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.

### 例子

#### 例子 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 * 10^4`
• `-2^31 <= nums[i] <= 2^31 - 1`
• `target is an integer from nums.`
• `At most 10^4 calls will be made to pick.`

### 解题思路

• 时间复杂度:
• constructor: `O(n)`
• pick: `O(1)`
• 空间复杂度: `O(n)`

GitHub 代码同步地址： 398.RandomPickIndex.cpp

GitHub: Leetcode-C++-Solution