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Author: assassint2017

Top K Frequent Elements/Top_K_Frequent_Elements.cpp

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+// Runtime: 20 ms, faster than 82.72% of C++ online submissions for Top K Frequent Elements.
+// Memory Usage: 11.5 MB, less than 48.54% of C++ online submissions for Top K Frequent Elements.
+
+class Solution 
+{
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) 
+	{
+		// 构建哈希字典
+		unordered_map<int, int> hashMap;
+		for (int item : nums)
+		{
+			if (hashMap.find(item) == hashMap.end())
+				hashMap.insert(pair<int, int>(item, 1));
+			else
+				++hashMap.at(item);
+		}
+
+		vector<pair<int, int>> vec, minHeap;
+		for (auto item : hashMap) vec.push_back(item);
+		for (int i = 0; i < k; ++i) minHeap.push_back(vec[i]);
+
+		// 定义匿名函数 这里是不允许使用auto的，所以面试的时候还是要谨慎使用
+		bool (*comp)(const pair<int, int>&, const pair<int, int>&) = \
+		[](const pair<int, int> &item1, const pair<int, int> &item2){return item1.second > item2.second;};
+
+		// 建立最小堆
+		make_heap(minHeap.begin(), minHeap.end(), comp);
+
+		// 遍历插入
+		for (int index = k; index < vec.size(); ++index)
+		{
+			if (minHeap[0].second < vec[index].second)
+			{
+				pop_heap(minHeap.begin(), minHeap.end(), comp);
+				minHeap.pop_back();
+
+				minHeap.push_back(vec[index]);
+				push_heap(minHeap.begin(), minHeap.end(), comp);
+			}
+		}
+
+		// 得到最终的结果
+		vector<int> res;
+		for (int i = 0; i < k; ++i)
+		{
+			pop_heap(minHeap.begin(), minHeap.end(), comp);
+			res.push_back(minHeap.back().first);
+			minHeap.pop_back();
+		}
+		return res;
+    }
+};
+
+
+// 优化代码
+// Runtime: 20 ms, faster than 82.72% of C++ online submissions for Top K Frequent Elements.
+// Memory Usage: 11.3 MB, less than 83.31% of C++ online submissions for Top K Frequent Elements.
+
+class Solution 
+{
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) 
+	{
+		// 构建哈希字典
+		unordered_map<int, int> hashMap;
+		for (int item : nums)
+		{
+			if (hashMap.find(item) == hashMap.end())
+				hashMap.insert(pair<int, int>(item, 1));
+			else
+				++hashMap.at(item);
+		}
+
+		vector<pair<int, int>> minHeap;
+		unordered_map<int, int>::iterator iter = hashMap.begin();
+		for (int i = 0; i < k; ++i, ++iter) minHeap.push_back(*iter);
+
+		// 定义匿名函数 这里是不允许使用auto的，所以面试的时候还是要谨慎使用
+		bool (*comp)(const pair<int, int>&, const pair<int, int>&) = \
+		[](const pair<int, int> &item1, const pair<int, int> &item2){return item1.second > item2.second;};
+
+		// 建立最小堆
+		make_heap(minHeap.begin(), minHeap.end(), comp);
+
+		// 遍历插入
+		for (; iter != hashMap.end(); ++iter)
+		{
+			if (minHeap[0].second < iter->second)
+			{
+				pop_heap(minHeap.begin(), minHeap.end(), comp);
+				minHeap.pop_back();
+
+				minHeap.push_back(*iter);
+				push_heap(minHeap.begin(), minHeap.end(), comp);
+			}
+		}
+
+		// 得到最终的结果
+		vector<int> res;
+		for (int i = 0; i < k; ++i)
+		{
+			pop_heap(minHeap.begin(), minHeap.end(), comp);
+			res.push_back(minHeap.back().first);
+			minHeap.pop_back();
+		}
+		return res;
+    }
+};