398. 随机数索引

class Solution 
{
    unordered_map<int,vector<int>> hash;// 对应的数 + /下标(可能有重复数)/
public:
    Solution(vector<int>& nums) 
    {
        for(int i = 0;i < nums.size();i ++) hash[nums[i]].push_back(i);
    }
    
    int pick(int target) 
    {
        int index = rand() % hash[target].size();
        return hash[target][index];
    }
};