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+# [381. Insert Delete GetRandom O(1) - Duplicates allowed](https://leetcode.com/problems/insert-delete-getrandom-o1-duplicates-allowed/)
+
+# 思路
+此题是[380. Insert Delete GetRandom O(1)](https://leetcode.com/problems/insert-delete-getrandom-o1/)的升级版，可参见[380题解](https://github.com/ShusenTang/LeetCode/blob/master/solutions/380.%20Insert%20Delete%20GetRandom%20O(1).md)。
+
+此题与380不同的是380不能有重复数字，而这道题可以有，那么就不能像380那样建立每个数字和其坐标的映射了，因为同一个可能有多个坐标。
+虽然不能索引到坐标，但我们可以索引到存放坐标的容器呀，用什么容器呢，一开始我想到vector, 但是在实现后面删除操作的时候没法O(1)，所以只好用`unordered_set`。
+
+其他的就和380思路大致差不多了，比如为了O(1)删除，需要变通一下：将待删除的元素和数组末元素交换一下，然后删除末尾元素。
+
+
+# C++
+``` C++
+class RandomizedCollection {
+private:
+    vector<int>data;
+    unordered_map<int, unordered_set<int>>idx_map;
+public:
+    /** Initialize your data structure here. */
+    RandomizedCollection() { }
+    
+    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
+    bool insert(int val) {
+        // if(!idx_map.count(val))
+        //    idx_map[val] = unordered_set<int>();
+     
+        idx_map[val].insert(data.size());   
+        data.push_back(val);
+        return idx_map[val].size() == 1;
+    }
+    
+    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
+    bool remove(int val) {
+        if(!idx_map.count(val) || idx_map[val].empty()) return false;
+        
+        int idx = *idx_map[val].begin();
+        
+        if(data[idx] != data.back()){ // 一定要加这个条件
+            data[idx] = data.back();
+            idx_map[data.back()].insert(idx);
+            idx_map[data.back()].erase(data.size() - 1);
+        }
+        
+        idx_map[val].erase(idx);
+        data.pop_back();
+        return true;
+    }
+    
+    /** Get a random element from the collection. */
+    int getRandom() {
+        return data[rand() % data.size()];
+    }
+};
+```
+