霍夫曼编码 | 贪心算法 2

霍夫曼编码是一种无损数据压缩算法。其思想是为输入字符分配可变长度代码，分配代码的长度基于相应字符的频率。 
分配给输入字符的可变长度代码是​​​​​​​前缀代码，意味着代码（位序列）的分配方式是分配给一个字符的代码不是分配给任何其他字符的代码的前缀。这就是霍夫曼编码如何确保在解码生成的比特流时没有歧义。 
让我们通过一个反例来理解前缀代码。设a、b、c、d四个字符，它们对应的变长码分别为00、01、0、1。这种编码会导致歧义，因为c的编码是a、b的编码前缀。如果压缩比特流是0001，解压输出可能是“cccd”或“ccb”或“acd”或“ab”。有关霍夫曼编码的应用， 
霍夫曼编码主要有两大部分

1. 从输入字符构建哈夫曼树。
2. 遍历哈夫曼树并为字符分配代码。

算法：

用于构造最优前缀码的方法称为霍夫曼编码。

 该算法以自下而上的方式构建树。我们可以用T来表示这棵树

让，|c| 是叶子的数量

|c| -1 是合并节点所需的操作数。Q 是构造二叉堆时可以使用的优先级队列。

Algorithm Huffman (c)
{n= |c| Q = c for i<-1 to n-1do{temp <- get node ()left (temp] Get_min (Q) right [temp] Get Min (Q)a = left [templ b = right [temp]F [temp]<- f[a] + [b]insert (Q, temp)}return Get_min (0)
}

构建哈夫曼树的步骤
输入是一组唯一字符及其出现频率，输出是哈夫曼树。 

1. 为每个唯一字符创建一个叶子节点，并构建一个所有叶子节点的最小堆（Min Heap用作优先队列。频率字段的值用于比较最小堆中的两个节点。最初，最不频繁的字符在根）
2. 从最小堆中提取频率最小的两个节点。
    
3. 创建一个新的内部节点，其频率等于两个节点频率的总和。将第一个提取的节点作为其左子节点，将另一个提取的节点作为其右子节点。将此节点添加到最小堆。
4. 重复步骤#2 和#3，直到堆只包含一个节点。剩下的节点是根节点，树就完成了。
   让我们通过一个例子来理解这个算法：

character   Frequencya            5b           9c           12d           13e           16f           45

步骤 1.构建一个包含 6 个节点的最小堆，其中每个节点代表具有单个节点的树的根。
步骤 2从最小堆中提取两个最小频率节点。添加频率为 5 + 9 = 14 的新内部节点。 
 

步骤 2 的图示

现在最小堆包含 5 个节点，其中 4 个节点是每个具有单个元素的树的根，一个堆节点是具有 3 个元素的树的根

character           Frequencyc               12d               13Internal Node         14e               16f                45

第三步：从堆中提取两个最小频率节点。添加频率为 12 + 13 = 25 的新内部节点
 

步骤 3 的图示

现在最小堆包含 4 个节点，其中 2 个节点是每个具有单个元素的树的根，两个堆节点是具有多个节点的树的根

character           Frequency
Internal Node          14e               16
Internal Node          25f               45

第四步：提取两个最小频率节点。添加频率为 14 + 16 = 30 的新内部节点
 

步骤 4 的图示

现在最小堆包含 3 个节点。

character          Frequency
Internal Node         25
Internal Node         30f               45 

步骤5：提取两个最小频率节点。添加频率为 25 + 30 = 55 的新内部节点
 

步骤 5 的图示

现在最小堆包含 2 个节点。

character     Frequencyf         45
Internal Node    55

第六步：提取两个最小频率节点。添加频率为 45 + 55 = 100 的新内部节点
 

步骤 6 的图示

现在最小堆只包含一个节点。

character      Frequency
Internal Node    100

由于堆只包含一个节点，算法到此为止。

从哈夫曼树打印代码的步骤：
遍历从根开始形成的树。维护一个辅助数组。在移动到左孩子的同时，将 0 写入数组。在移动到右孩子的同时，将 1 写入数组。遇到叶节点时打印数组。
 

从 HuffmanTree 打印代码的步骤

代码如下：

character   code-wordf          0c          100d          101a          1100b          1101e          111

下面是上述方法的实现： 

// C++ program for Huffman Coding
#include <cstdlib>
#include <iostream>
using namespace std;// This constant can be avoided by explicitly
// calculating height of Huffman Tree
#define MAX_TREE_HT 100// A Huffman tree node
struct MinHeapNode {// One of the input characterschar data;// Frequency of the characterunsigned freq;// Left and right child of this nodestruct MinHeapNode *left, *right;
};// A Min Heap: Collection of
// min-heap (or Huffman tree) nodes
struct MinHeap {// Current size of min heapunsigned size;// capacity of min heapunsigned capacity;// Array of minheap node pointersstruct MinHeapNode** array;
};// A utility function allocate a new
// min heap node with given character
// and frequency of the character
struct MinHeapNode* newNode(char data, unsigned freq)
{struct MinHeapNode* temp = (struct MinHeapNode*)malloc(sizeof(struct MinHeapNode));temp->left = temp->right = NULL;temp->data = data;temp->freq = freq;return temp;
}// A utility function to create
// a min heap of given capacity
struct MinHeap* createMinHeap(unsigned capacity){struct MinHeap* minHeap= (struct MinHeap*)malloc(sizeof(struct MinHeap));// current size is 0minHeap->size = 0;minHeap->capacity = capacity;minHeap->array = (struct MinHeapNode**)malloc(minHeap->capacity * sizeof(struct MinHeapNode*));return minHeap;
}// A utility function to
// swap two min heap nodes
void swapMinHeapNode(struct MinHeapNode** a,struct MinHeapNode** b){struct MinHeapNode* t = *a;*a = *b;*b = t;
}// The standard minHeapify function.
void minHeapify(struct MinHeap* minHeap, int idx){int smallest = idx;int left = 2 * idx + 1;int right = 2 * idx + 2;if (left < minHeap->size&& minHeap->array[left]->freq< minHeap->array[smallest]->freq)smallest = left;if (right < minHeap->size&& minHeap->array[right]->freq< minHeap->array[smallest]->freq)smallest = right;if (smallest != idx) {swapMinHeapNode(&minHeap->array[smallest],&minHeap->array[idx]);minHeapify(minHeap, smallest);}
}// A utility function to check
// if size of heap is 1 or not
int isSizeOne(struct MinHeap* minHeap)
{return (minHeap->size == 1);
}// A standard function to extract
// minimum value node from heap
struct MinHeapNode* extractMin(struct MinHeap* minHeap){struct MinHeapNode* temp = minHeap->array[0];minHeap->array[0] = minHeap->array[minHeap->size - 1];--minHeap->size;minHeapify(minHeap, 0);return temp;
}// A utility function to insert
// a new node to Min Heap
void insertMinHeap(struct MinHeap* minHeap,struct MinHeapNode* minHeapNode){++minHeap->size;int i = minHeap->size - 1;while (i&& minHeapNode->freq< minHeap->array[(i - 1) / 2]->freq) {minHeap->array[i] = minHeap->array[(i - 1) / 2];i = (i - 1) / 2;}minHeap->array[i] = minHeapNode;
}// A standard function to build min heap
void buildMinHeap(struct MinHeap* minHeap){int n = minHeap->size - 1;int i;for (i = (n - 1) / 2; i >= 0; --i)minHeapify(minHeap, i);
}// A utility function to print an array of size n
void printArr(int arr[], int n)
{int i;for (i = 0; i < n; ++i)cout << arr[i];cout << "\n";
}// Utility function to check if this node is leaf
int isLeaf(struct MinHeapNode* root){return !(root->left) && !(root->right);
}// Creates a min heap of capacity
// equal to size and inserts all character of
// data[] in min heap. Initially size of
// min heap is equal to capacity
struct MinHeap* createAndBuildMinHeap(char data[],int freq[], int size){struct MinHeap* minHeap = createMinHeap(size);for (int i = 0; i < size; ++i)minHeap->array[i] = newNode(data[i], freq[i]);minHeap->size = size;buildMinHeap(minHeap);return minHeap;
}// The main function that builds Huffman tree
struct MinHeapNode* buildHuffmanTree(char data[],int freq[], int size){struct MinHeapNode *left, *right, *top;// Step 1: Create a min heap of capacity// equal to size. Initially, there are// modes equal to size.struct MinHeap* minHeap= createAndBuildMinHeap(data, freq, size);// Iterate while size of heap doesn't become 1while (!isSizeOne(minHeap)) {// Step 2: Extract the two minimum// freq items from min heapleft = extractMin(minHeap);right = extractMin(minHeap);// Step 3: Create a new internal// node with frequency equal to the// sum of the two nodes frequencies.// Make the two extracted node as// left and right children of this new node.// Add this node to the min heap// '$' is a special value for internal nodes, not// usedtop = newNode('$', left->freq + right->freq);top->left = left;top->right = right;insertMinHeap(minHeap, top);}// Step 4: The remaining node is the// root node and the tree is complete.return extractMin(minHeap);
}// Prints huffman codes from the root of Huffman Tree.
// It uses arr[] to store codes
void printCodes(struct MinHeapNode* root, int arr[],int top){// Assign 0 to left edge and recurif (root->left) {arr[top] = 0;printCodes(root->left, arr, top + 1);}// Assign 1 to right edge and recurif (root->right) {arr[top] = 1;printCodes(root->right, arr, top + 1);}// If this is a leaf node, then// it contains one of the input// characters, print the character// and its code from arr[]if (isLeaf(root)) {cout << root->data << ": ";printArr(arr, top);}
}// The main function that builds a
// Huffman Tree and print codes by traversing
// the built Huffman Tree
void HuffmanCodes(char data[], int freq[], int size){// Construct Huffman Treestruct MinHeapNode* root= buildHuffmanTree(data, freq, size);// Print Huffman codes using// the Huffman tree built aboveint arr[MAX_TREE_HT], top = 0;printCodes(root, arr, top);
}// Driver code
int main()
{char arr[] = { 'a', 'b', 'c', 'd', 'e', 'f' };int freq[] = { 5, 9, 12, 13, 16, 45 };int size = sizeof(arr) / sizeof(arr[0]);HuffmanCodes(arr, freq, size);return 0;
}

// C++(STL) program for Huffman Coding with STL
#include <bits/stdc++.h>
using namespace std;// A Huffman tree node
struct MinHeapNode {// One of the input characterschar data;// Frequency of the characterunsigned freq;// Left and right childMinHeapNode *left, *right;MinHeapNode(char data, unsigned freq){left = right = NULL;this->data = data;this->freq = freq;}
};// For comparison of
// two heap nodes (needed in min heap)
struct compare {bool operator()(MinHeapNode* l, MinHeapNode* r){return (l->freq > r->freq);}
};// Prints huffman codes from
// the root of Huffman Tree.
void printCodes(struct MinHeapNode* root, string str)
{if (!root)return;if (root->data != '$')cout << root->data << ": " << str << "\n";printCodes(root->left, str + "0");printCodes(root->right, str + "1");
}// The main function that builds a Huffman Tree and
// print codes by traversing the built Huffman Tree
void HuffmanCodes(char data[], int freq[], int size)
{struct MinHeapNode *left, *right, *top;// Create a min heap & inserts all characters of data[]priority_queue<MinHeapNode*, vector<MinHeapNode*>,compare>minHeap;for (int i = 0; i < size; ++i)minHeap.push(new MinHeapNode(data[i], freq[i]));// Iterate while size of heap doesn't become 1while (minHeap.size() != 1) {// Extract the two minimum// freq items from min heapleft = minHeap.top();minHeap.pop();right = minHeap.top();minHeap.pop();// Create a new internal node with// frequency equal to the sum of the// two nodes frequencies. Make the// two extracted node as left and right children// of this new node. Add this node// to the min heap '$' is a special value// for internal nodes, not usedtop = new MinHeapNode('$',left->freq + right->freq);top->left = left;top->right = right;minHeap.push(top);}// Print Huffman codes using// the Huffman tree built aboveprintCodes(minHeap.top(), "");
}// Driver Code
int main()
{char arr[] = { 'a', 'b', 'c', 'd', 'e', 'f' };int freq[] = { 5, 9, 12, 13, 16, 45 };int size = sizeof(arr) / sizeof(arr[0]);HuffmanCodes(arr, freq, size);return 0;
}// This code is contributed by Aditya Goel

Output

f: 0
c: 100
d: 101
a: 1100
b: 1101
e: 111

时间复杂度： O(nlogn)，其中 n 是唯一字符的数量。如果有 n 个节点，则调用 extractMin() 2*(n – 1) 次。extractMin() 在调用 minHeapify() 时需要 O(logn) 时间。因此，整体复杂度为 O(nlogn)。
如果输入数组已排序，则存在线性时间算法。我们很快就会在下一篇文章中讨论这个问题。

霍夫曼编码的应用：

1. 它们用于传输传真和文本。
2. 它们被传统的压缩格式使用，如 PKZIP、GZIP 等。
3. JPEG、PNG 和 MP3 等多媒体编解码器使用霍夫曼编码（更准确地说是前缀代码）。

 它在有一系列频繁出现的字符的情况下很有用。