solve lab 9

labs/hash_tables/test_longest_consecutive_sequence.cpp

@@ -1,23 +1,113 @@
 #include <iostream>
 #include <vector>
+#include <list>
+
+// A minimal separate-chaining HashSet for integers.
+class MyHashSet {
+private:
+    std::vector< std::list<int> > table;
+    int num_elements;
+    static const int DEFAULT_SIZE = 16;
+    static const double LOAD_FACTOR_THRESHOLD;
+
+    int hashFunction(int key) const {
+        int h = key < 0 ? -key : key;  // handle negative
+        return h % (int)table.size();
+    }
+
+    void rehash() {
+        int newSize = (int)table.size() * 2;
+        std::vector< std::list<int> > newTable(newSize);
+
+        for (auto &chain : table) {
+            for (int val : chain) {
+                int h = (val < 0 ? -val : val) % newSize;
+                newTable[h].push_back(val);
+            }
+        }
+        table.swap(newTable);
+    }
+
+    void rehashIfNeeded() {
+        double load_factor = (double)num_elements / (double)table.size();
+        if (load_factor > LOAD_FACTOR_THRESHOLD) {
+            rehash();
+        }
+    }
+
+public:
+    MyHashSet(int initialSize = DEFAULT_SIZE)
+        : table(initialSize), num_elements(0) {}
+
+    bool find(int key) const {
+        int h = hashFunction(key);
+        for (int val : table[h]) {
+            if (val == key) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    void insert(int key) {
+        if (!find(key)) {
+            int h = hashFunction(key);
+            table[h].push_back(key);
+            num_elements++;
+            rehashIfNeeded();
+        }
+    }
+};
+
+const double MyHashSet::LOAD_FACTOR_THRESHOLD = 1.0;
 
 int longestConsecutive(std::vector<int>& nums) {
+    if (nums.empty()) return 0;
+
+    // Insert all numbers into the custom hash set
+    MyHashSet set;
+    for (int n : nums) {
+        set.insert(n);
+    }
+
+    int longestStreak = 0;
+    // For each number, only count a sequence if it's a "start"
+    // i.e., if (n-1) is NOT in the set
+    for (int n : nums) {
+        if (!set.find(n - 1)) {
+            // n is the start of a consecutive sequence
+            int currentNum = n;
+            int currentStreak = 1;
+
+            while (set.find(currentNum + 1)) {
+                currentNum++;
+                currentStreak++;
+            }
+            if (currentStreak > longestStreak) {
+                longestStreak = currentStreak;
+            }
+        }
+    }
+
+    return longestStreak;
 }
 
 int main() {
-	//std::vector<int> nums = {100, 4, 200, 1, 3, 2};
-	std::vector<int> nums = {100, 4, 200, 1, 3, 2, 2, 2, 2, 3};
-	//std::vector<int> nums = {100, 4, 200, 1, 3, 2, 5, 6};
-	//std::vector<int> nums = {0,3,7,2,5,8,4,6,0,1};
-	//std::vector<int> nums = {100, 4, 200, 201, 202, 203, 205, 204, 1, 3, 2};
-	//std::vector<int> nums = {-3,0,1,2,3,-2,-1,-5};
-	int size = nums.size();
-	std::cout<< "for vector {";
-	for(int i=0;i<size-1;i++){
-		std::cout<< nums[i] << ",";
-	}
-	std::cout<< nums[size-1] << "}" <<std::endl;
-	int length = longestConsecutive(nums);
-	std::cout << "The length of the longest consecutive sequence is: " << length << std::endl;
-	return 0;
+    // You can change these test vectors to experiment:
+    //std::vector<int> nums = {100, 4, 200, 1, 3, 2};
+    //std::vector<int> nums = {0,3,7,2,5,8,4,6,0,1};
+    std::vector<int> nums = {100, 4, 200, 1, 3, 2, 2, 2, 2, 3};
+
+    int size = nums.size();
+    std::cout << "for vector {";
+    for(int i = 0; i < size; i++){
+        if(i < size - 1) std::cout << nums[i] << ",";
+        else std::cout << nums[i];
+    }
+    std::cout << "}\n";
+
+    int length = longestConsecutive(nums);
+    std::cout << "The length of the longest consecutive sequence is: "
+              << length << std::endl;
+    return 0;
 }