Huffman coding

Author: coderick14

huffman.cpp

@@ -0,0 +1,81 @@
+#include <bits/stdc++.h>
+#define ll long long
+using namespace std;
+
+struct node {
+    char letter;
+    ll freq;
+    struct node* left;
+    struct node* right;
+
+    node(char letter, ll freq) {
+        this->letter = letter;
+        this->freq = freq;
+        this->left = this->right = NULL;
+    }
+};
+
+/*
+ * Comparison function for min_heap
+ */
+class Prioritize {
+    public:
+        int operator() (struct node* a, struct node* b) {
+            return a->freq > b->freq;
+        }
+};
+
+/*
+ * function to build the huffman tree
+ * time complexity : O(nlogn) as each heap operation takes O(logn) time
+ */ 
+struct node* build_huffman(vector<pair<char,ll> > &chars, int n) {
+    int i;
+    priority_queue<node*, vector<node*>, Prioritize> min_heap;
+    for(i=0; i<n; i++) {
+        min_heap.push(new node(chars[i].first, chars[i].second));
+    }
+    for(i=0; i<n-1; i++) {
+        struct node* left_child = min_heap.top();
+        min_heap.pop();
+        struct node* right_child = min_heap.top();
+        min_heap.pop();
+        struct node* new_node = new node('$', left_child->freq + right_child->freq);
+        new_node->left = left_child;
+        new_node->right = right_child;
+        min_heap.push(new_node);
+    }
+    struct node* root = min_heap.top();
+    min_heap.pop();
+    return root;
+}
+
+/*
+ * function to print huffman codes for the characters
+ * time complexity : O(n) as there are 2n-1 nodes in the tree (n leaf nodes and n-1 internal nodes)
+ *                   and each node is visited only once
+ */
+void print_huffman_codes(struct node* root, string str) {
+    if(root == NULL) {
+        return;
+    }
+    if(root->letter == '$') {
+        print_huffman_codes(root->left, str+"0");
+        print_huffman_codes(root->right, str+"1");
+    }
+    else {
+        cout<<root->letter<<" : "<<str<<endl;
+    }
+}
+
+int main() {
+    int i,n;
+    cin>>n; //number of characters
+    vector<pair<char,ll> > chars(n);
+    for(i=0; i<n; i++) {
+        cin>>chars[i].first>>chars[i].second;
+    }
+    struct node* root = build_huffman(chars, n);
+    print_huffman_codes(root, "");
+    return 0;
+}