Random Pick Index

题目来源
给一个数组存在重复元素，给一个元素，随机选取数组中该元素所在位置。
我想着直接用哈希，存储每个元素对应的位置，然后内存溢出了。

class Solution {
public:
    Solution(vector nums) {
        int n = nums.size();
        for (int i=0; i> maps;
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);
 */

难道还能不存储位置？看了下tags，蓄水池抽样。就是未知数目的元素，取样k个，然后前k个都取。假设取到第p个的时候，取该数的概率是k/p，然后随机替换k个中的一个。

class Solution {
public:
    Solution(vector nums) {
        this->nums = nums;
    }
    
    int pick(int target) {
        int n = nums.size();
        int res = 0, cnt = 0;
        for (int i=0; i nums;
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);
 */