Kth largest element in an array

Author: volodimyr

215.kth_largest_element_in_an_array/largest.go

@@ -0,0 +1,55 @@
+// 215. Kth largest element in an array
+// Topics: 'Heap (Priority Queue)', 'Sorting', 'Quickselect', 'Divide and Conquer', 'Array'
+// Level: 'Medium'
+
+// Given an integer array nums and an integer k, return the kth largest element in the array.
+
+// Note that it is the kth largest element in the sorted order, not the kth distinct element.
+
+// Can you solve it without sorting?
+
+// Example 1:
+
+// Input: nums = [3,2,1,5,6,4], k = 2
+// Output: 5
+
+// Example 2:
+
+// Input: nums = [3,2,3,1,2,4,5,5,6], k = 4
+// Output: 4
+
+// Constraints:
+
+//     1 <= k <= nums.length <= 105
+//     -104 <= nums[i] <= 104
+
+package kthlargestelementinanarray
+
+func findKthLargest(nums []int, max int) int {
+	arr := []int{}
+	for i, n := range nums {
+		if i < max {
+			arr = append(arr, n)
+			j := len(arr) - 1
+			for j > 0 && arr[j] < arr[(j-1)/2] {
+				arr[j], arr[(j-1)/2] = arr[(j-1)/2], arr[j]
+				j = (j - 1) / 2
+			}
+		} else if n > arr[0] {
+			arr[0] = n
+			j := 0
+			for 2*j+1 < len(arr) {
+				if 2*j+2 < len(arr) && arr[2*j+2] < arr[2*j+1] && arr[j] > arr[2*j+2] {
+					arr[j], arr[2*j+2] = arr[2*j+2], arr[j]
+					j = 2*j + 2
+				} else if arr[j] > arr[2*j+1] {
+					arr[j], arr[2*j+1] = arr[2*j+1], arr[j]
+					j = 2*j + 1
+				} else {
+					break
+				}
+			}
+		}
+	}
+	return arr[0]
+}

215.kth_largest_element_in_an_array/largest_test.go

@@ -0,0 +1,115 @@
+package kthlargestelementinanarray
+
+import "testing"
+
+func TestFindKthLargest(t *testing.T) {
+	tests := []struct {
+		name string
+		nums []int
+		k    int
+		want int
+	}{
+		{
+			name: "example 1",
+			nums: []int{3, 2, 1, 5, 6, 4},
+			k:    2,
+			want: 5,
+		},
+		{
+			name: "example 2",
+			nums: []int{3, 2, 3, 1, 2, 4, 5, 5, 6},
+			k:    4,
+			want: 4,
+		},
+		{
+			name: "single element",
+			nums: []int{1},
+			k:    1,
+			want: 1,
+		},
+		{
+			name: "k equals 1",
+			nums: []int{3, 2, 1, 5, 6, 4},
+			k:    1,
+			want: 6,
+		},
+		{
+			name: "k equals array length",
+			nums: []int{3, 2, 1, 5, 6, 4},
+			k:    6,
+			want: 1,
+		},
+		{
+			name: "all same elements",
+			nums: []int{5, 5, 5, 5, 5},
+			k:    3,
+			want: 5,
+		},
+		{
+			name: "negative numbers",
+			nums: []int{-1, -2, -3, -4, -5},
+			k:    2,
+			want: -2,
+		},
+		{
+			name: "mixed positive and negative",
+			nums: []int{-3, 2, -1, 5, -6, 4},
+			k:    3,
+			want: 2,
+		},
+		{
+			name: "two elements",
+			nums: []int{2, 1},
+			k:    1,
+			want: 2,
+		},
+		{
+			name: "two elements k=2",
+			nums: []int{2, 1},
+			k:    2,
+			want: 1,
+		},
+		{
+			name: "already sorted ascending",
+			nums: []int{1, 2, 3, 4, 5},
+			k:    2,
+			want: 4,
+		},
+		{
+			name: "already sorted descending",
+			nums: []int{5, 4, 3, 2, 1},
+			k:    2,
+			want: 4,
+		},
+		{
+			name: "duplicates at boundaries",
+			nums: []int{1, 1, 1, 2, 2, 3, 3, 3, 3},
+			k:    4,
+			want: 3,
+		},
+		{
+			name: "large values",
+			nums: []int{10000, 9999, 9998, 9997, 9996},
+			k:    3,
+			want: 9998,
+		},
+		{
+			name: "small values",
+			nums: []int{-10000, -9999, -9998, -9997, -9996},
+			k:    3,
+			want: -9998,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			numsCopy := make([]int, len(tt.nums))
+			copy(numsCopy, tt.nums)
+
+			got := findKthLargest(numsCopy, tt.k)
+			if got != tt.want {
+				t.Errorf("findKthLargest(%v, %d) = %d, want %d", tt.nums, tt.k, got, tt.want)
+			}
+		})
+	}
+}