[Arrays & Hashing][Longest Consecutive Sequence] - Implement solution for Leetcode 128

1_arrays_and_hashing/9_longest_consecutive_sequence/solution.go

@@ -0,0 +1,141 @@
+package longestconsecutivesequence
+
+import (
+	"sort"
+)
+
+// Time complexity: O(n^2)
+// Space complexity: O(1)
+func LongestConsecutiveBruteForce1(nums []int) int {
+	maxLen := 0
+
+	for _, num := range nums {
+		current := num
+		length := 1
+
+		for {
+			found := false
+			for _, n := range nums {
+				if n == current+1 {
+					found = true
+					current++
+					length++
+					break
+				}
+			}
+			if !found {
+				break
+			}
+		}
+
+		if length > maxLen {
+			maxLen = length
+		}
+	}
+
+	return maxLen
+}
+
+// Time complexity: O(n^2)
+// Space complexity: O(n)
+func LongestConsecutiveBruteForce2(nums []int) int {
+	res := 0
+	store := make(map[int]struct{})
+	for _, num := range nums {
+		store[num] = struct{}{}
+	}
+
+	for _, num := range nums {
+		streak, curr := 0, num
+		for _, ok := store[curr]; ok; _, ok = store[curr] {
+			streak++
+			curr++
+		}
+		if streak > res {
+			res = streak
+		}
+	}
+	return res
+}
+
+// Time complexity: O(n log n) due to sorting
+// Space complexity: O(1) or O(n) depending on sorting algorithm implementation
+func LongestConsecutiveSort(nums []int) int {
+	if len(nums) == 0 {
+		return 0
+	}
+
+	sort.Ints(nums)
+	maxLen := 1
+	currLen := 1
+
+	for i := 1; i < len(nums); i++ {
+		if nums[i] == nums[i-1] {
+			continue // skip duplicates
+		}
+		if nums[i] == nums[i-1]+1 {
+			currLen++
+		} else {
+			currLen = 1
+		}
+		if currLen > maxLen {
+			maxLen = currLen
+		}
+	}
+	return maxLen
+}
+
+// Time complexity: O(n)
+// Space complexity: O(n)
+func LongestConsecutiveHashSet(nums []int) int {
+	set := make(map[int]bool)
+	for _, num := range nums {
+		set[num] = true
+	}
+
+	maxLen := 0
+	for num := range set {
+		// Only check starts of sequences
+		if !set[num-1] {
+			current := num
+			length := 1
+
+			for set[current+1] {
+				current++
+				length++
+			}
+			if length > maxLen {
+				maxLen = length
+			}
+		}
+	}
+	return maxLen
+}
+
+// Time complexity: O(n)
+// Space complexity: O(n)
+func LongestConsecutiveHashMap(nums []int) int {
+	if len(nums) == 0 {
+		return 0
+	}
+
+	lookup := make(map[int]int)
+	maxLen := 0
+
+	for _, num := range nums {
+		if _, exists := lookup[num]; !exists {
+			left := lookup[num-1]
+			right := lookup[num+1]
+			length := left + right + 1
+
+			lookup[num] = length
+			lookup[num-left] = length
+			lookup[num+right] = length
+
+			if length > maxLen {
+				maxLen = length
+			}
+		}
+	}
+	return maxLen
+}

1_arrays_and_hashing/9_longest_consecutive_sequence/solution_test.go

@@ -0,0 +1,56 @@
+package longestconsecutivesequence
+
+import "testing"
+
+func TestLongestConsecutive(t *testing.T) {
+	cases := []struct {
+		name     string
+		fn       func([]int) int
+		nums     []int
+		expected int
+	}{
+		// LongestConsecutiveBruteForce1
+		{"Brute Force - Example 1", LongestConsecutiveBruteForce1, []int{2, 20, 4, 10, 3, 4, 5}, 4},
+		{"Brute Force - Example 2", LongestConsecutiveBruteForce1, []int{0, 3, 2, 5, 4, 6, 1, 1}, 7},
+		{"Brute Force - Empty", LongestConsecutiveBruteForce1, []int{}, 0},
+		{"Brute Force - Single Element", LongestConsecutiveBruteForce1, []int{42}, 1},
+		{"Brute Force - All Same", LongestConsecutiveBruteForce1, []int{7, 7, 7}, 1},
+
+		// LongestConsecutiveBruteForce2
+		{"Brute Force - Example 1", LongestConsecutiveBruteForce2, []int{2, 20, 4, 10, 3, 4, 5}, 4},
+		{"Brute Force - Example 2", LongestConsecutiveBruteForce2, []int{0, 3, 2, 5, 4, 6, 1, 1}, 7},
+		{"Brute Force - Empty", LongestConsecutiveBruteForce2, []int{}, 0},
+		{"Brute Force - Single Element", LongestConsecutiveBruteForce2, []int{42}, 1},
+		{"Brute Force - All Same", LongestConsecutiveBruteForce2, []int{7, 7, 7}, 1},
+
+		// LongestConsecutiveSort
+		{"Sort - Example 1", LongestConsecutiveSort, []int{2, 20, 4, 10, 3, 4, 5}, 4},
+		{"Sort - Example 2", LongestConsecutiveSort, []int{0, 3, 2, 5, 4, 6, 1, 1}, 7},
+		{"Sort - Empty", LongestConsecutiveSort, []int{}, 0},
+		{"Sort - Single Element", LongestConsecutiveSort, []int{42}, 1},
+		{"Sort - All Same", LongestConsecutiveSort, []int{7, 7, 7}, 1},
+
+		// LongestConsecutiveHashSet
+		{"Hash Set - Example 1", LongestConsecutiveHashSet, []int{2, 20, 4, 10, 3, 4, 5}, 4},
+		{"Hash Set - Example 2", LongestConsecutiveHashSet, []int{0, 3, 2, 5, 4, 6, 1, 1}, 7},
+		{"Hash Set - Empty", LongestConsecutiveHashSet, []int{}, 0},
+		{"Hash Set - Single Element", LongestConsecutiveHashSet, []int{42}, 1},
+		{"Hash Set - All Same", LongestConsecutiveHashSet, []int{7, 7, 7}, 1},
+
+		// LongestConsecutiveHashMap
+		{"Hash Map - Example 1", LongestConsecutiveHashMap, []int{2, 20, 4, 10, 3, 4, 5}, 4},
+		{"Hash Map - Example 2", LongestConsecutiveHashMap, []int{0, 3, 2, 5, 4, 6, 1, 1}, 7},
+		{"Hash Map - Empty", LongestConsecutiveHashMap, []int{}, 0},
+		{"Hash Map - Single Element", LongestConsecutiveHashMap, []int{42}, 1},
+		{"Hash Map - All Same", LongestConsecutiveHashMap, []int{7, 7, 7}, 1},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := c.fn(c.nums)
+			if got != c.expected {
+				t.Errorf("%s failed: expected %d, got %d", c.name, c.expected, got)
+			}
+		})
+	}
+}