[Arrays & Hashing][Two Sum] - Implement solution for Leetcode 1 (#8)

Author: javydevx

1_arrays_and_hashing/3_two_sum/solution.go

@@ -0,0 +1,79 @@
+package twosum
+
+import "sort"
+
+// Time complexity: O(n^2)
+// Space complexity: O(1)
+func TwoSumBruteForce(nums []int, target int) []int {
+	for i := 0; i < len(nums); i++ {
+		for j := i + 1; j < len(nums); j++ {
+			if nums[i]+nums[j] == target {
+				return []int{i, j}
+			}
+		}
+	}
+	return nil
+}
+
+type Pair struct {
+	val int
+	idx int
+}
+
+// Time complexity: O(n log n)
+// Space complexity: O(n)
+func TwoSumSorted(nums []int, target int) []int {
+	pairs := make([]Pair, len(nums))
+	for i, v := range nums {
+		pairs[i] = Pair{val: v, idx: i}
+	}
+	sort.Slice(pairs, func(i, j int) bool {
+		return pairs[i].val < pairs[j].val
+	})
+
+	left, right := 0, len(pairs)-1
+	for left < right {
+		sum := pairs[left].val + pairs[right].val
+		if sum == target {
+			return []int{pairs[left].idx, pairs[right].idx}
+		} else if sum < target {
+			left++
+		} else {
+			right--
+		}
+	}
+	return nil
+}
+
+// Time complexity: O(n)
+// Space complexity: O(n)
+func TwoSumMapTwoPass(nums []int, target int) []int {
+	// First pass: build map
+	m := make(map[int]int) // value -> index
+	for i, num := range nums {
+		m[num] = i
+	}
+
+	// Second pass: check complements
+	for i, num := range nums {
+		complement := target - num
+		if j, ok := m[complement]; ok && i != j {
+			return []int{i, j}
+		}
+	}
+	return nil
+}
+
+// Time complexity: O(n)
+// Space complexity: O(n)
+func TwoSumMapOnePass(nums []int, target int) []int {
+	m := make(map[int]int) // value -> index
+	for i, num := range nums {
+		complement := target - num
+		if j, ok := m[complement]; ok {
+			return []int{j, i}
+		}
+		m[num] = i
+	}
+	return nil
+}

1_arrays_and_hashing/3_two_sum/solution_test.go

@@ -0,0 +1,57 @@
+package twosum
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestTwoSum(t *testing.T) {
+	cases := []struct {
+		name     string
+		fn       func([]int, int) []int
+		nums     []int
+		target   int
+		expected []int
+	}{
+		// TwoSumBruteForce
+		{"Brute Force - Valid Pair", TwoSumBruteForce, []int{2, 7, 11, 15}, 9, []int{0, 1}},
+		{"Brute Force - No Pair", TwoSumBruteForce, []int{1, 2, 3}, 7, nil},
+		{"Brute - Empty", TwoSumBruteForce, []int{}, 0, nil},
+		{"Brute - Single Element", TwoSumBruteForce, []int{5}, 5, nil},
+		{"Brute - Negative Numbers", TwoSumBruteForce, []int{-3, 4, 3, 90}, 0, []int{0, 2}},
+		{"Brute - Duplicates", TwoSumBruteForce, []int{3, 3}, 6, []int{0, 1}},
+
+		// Sorting (Two Pointer)
+		{"Sorted - Valid Pair", TwoSumSorted, []int{2, 7, 11, 15}, 9, []int{0, 1}},
+		{"Sorted - No Pair", TwoSumSorted, []int{1, 2, 3}, 7, nil},
+		{"Sorted - Empty", TwoSumSorted, []int{}, 0, nil},
+		{"Sorted - Single Element", TwoSumSorted, []int{5}, 5, nil},
+		{"Sorted - Negative Numbers", TwoSumSorted, []int{-3, 4, 3, 90}, 0, []int{0, 2}},
+		{"Sorted - Duplicates", TwoSumSorted, []int{3, 3}, 6, []int{0, 1}},
+
+		// Map Two-Pass
+		{"MapTwoPass - Valid Pair", TwoSumMapTwoPass, []int{2, 7, 11, 15}, 9, []int{0, 1}},
+		{"MapTwoPass - No Pair", TwoSumMapTwoPass, []int{1, 2, 3}, 7, nil},
+		{"MapTwoPass - Empty", TwoSumMapTwoPass, []int{}, 0, nil},
+		{"MapTwoPass - Single", TwoSumMapTwoPass, []int{5}, 5, nil},
+		{"MapTwoPass - Negative", TwoSumMapTwoPass, []int{-3, 4, 3, 90}, 0, []int{0, 2}},
+		{"MapTwoPass - Duplicates", TwoSumMapTwoPass, []int{3, 3}, 6, []int{0, 1}},
+
+		// Map One-Pass
+		{"MapOnePass - Valid Pair", TwoSumMapOnePass, []int{2, 7, 11, 15}, 9, []int{0, 1}},
+		{"MapOnePass - No Pair", TwoSumMapOnePass, []int{1, 2, 3}, 7, nil},
+		{"MapOnePass - Empty", TwoSumMapOnePass, []int{}, 0, nil},
+		{"MapOnePass - Single", TwoSumMapOnePass, []int{5}, 5, nil},
+		{"MapOnePass - Negative", TwoSumMapOnePass, []int{-3, 4, 3, 90}, 0, []int{0, 2}},
+		{"MapOnePass - Duplicates", TwoSumMapOnePass, []int{3, 3}, 6, []int{0, 1}},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := c.fn(c.nums, c.target)
+			if !reflect.DeepEqual(got, c.expected) {
+				t.Errorf("%s failed: expected %v, got %v", c.name, c.expected, got)
+			}
+		})
+	}
+}