feat: implementation of find kth element in array (TheAlgorithms#472) (…

…TheAlgorithms#473) * feat: implementation of find kth element in array (TheAlgorithms#472) * feat: implementation of find kth element in array * feat: implementation of find kth element in array * Updated Documentation in README.md Co-authored-by: github-action <${GITHUB_ACTOR}@users.noreply.github.com> Co-authored-by: Rak Laptudirm <[email protected]>
pengchant · Mar 11, 2022 · b875ba7 · b875ba7
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diff --git a/README.md b/README.md
@@ -545,8 +545,10 @@ Read our [Contribution Guidelines](CONTRIBUTING.md) before you contribute.
 ##### Functions:
 
 1. [`Abs`](./math/abs.go#L11):  Abs returns absolute value
-2. [`IsPowOfTwoUseLog`](./math/checkisnumberpoweroftwo.go#L10):  IsPowOfTwoUseLog This function checks if a number is a power of two using the logarithm. The limiting degree can be from 0 to 63. See alternatives in the binary package.
-3. [`Phi`](./math/eulertotient.go#L5):  Phi is the Euler totient function. This function computes the number of numbers less then n that are coprime with n.
+2. [`FindKthMax`](./math/kthnumber.go#L11):  FindKthMax returns the kth large element given an integer slice with nil `error` if found and returns -1 with `error` `search.ErrNotFound` if not found. NOTE: The `nums` slice gets mutated in the process.
+3. [`FindKthMin`](./math/kthnumber.go#L19):  FindKthMin returns kth small element given an integer slice with nil `error` if found and returns -1 with `error` `search.ErrNotFound` if not found. NOTE: The `nums` slice gets mutated in the process.
+4. [`IsPowOfTwoUseLog`](./math/checkisnumberpoweroftwo.go#L10):  IsPowOfTwoUseLog This function checks if a number is a power of two using the logarithm. The limiting degree can be from 0 to 63. See alternatives in the binary package.
+5. [`Phi`](./math/eulertotient.go#L5):  Phi is the Euler totient function. This function computes the number of numbers less then n that are coprime with n.
 
 ---
 </details><details>
@@ -861,13 +863,14 @@ Read our [Contribution Guidelines](CONTRIBUTING.md) before you contribute.
 6. [`InsertionSort`](./sort/insertionsort.go#L3): No description provided.
 7. [`MergeIter`](./sort/mergesort.go#L51): No description provided.
 8. [`Mergesort`](./sort/mergesort.go#L37): Mergesort Perform mergesort on a slice of ints
-9. [`Pigeonhole`](./sort/pigeonholesort.go#L12):  Pigeonhole sorts a slice using pigeonhole sorting algorithm.
-10. [`QuickSort`](./sort/quicksort.go#L37):  QuickSort Sorts the entire array
-11. [`QuickSortRange`](./sort/quicksort.go#L24):  QuickSortRange Sorts the specified range within the array
-12. [`RadixSort`](./sort/radixsort.go#L35): No description provided.
-13. [`SelectionSort`](./sort/selectionsort.go#L3): No description provided.
-14. [`ShellSort`](./sort/shellsort.go#L3): No description provided.
-15. [`SimpleSort`](./sort/simplesort.go#L11): No description provided.
+9. [`Partition`](./sort/quicksort.go#L10): No description provided.
+10. [`Pigeonhole`](./sort/pigeonholesort.go#L12):  Pigeonhole sorts a slice using pigeonhole sorting algorithm.
+11. [`QuickSort`](./sort/quicksort.go#L37):  QuickSort Sorts the entire array
+12. [`QuickSortRange`](./sort/quicksort.go#L24):  QuickSortRange Sorts the specified range within the array
+13. [`RadixSort`](./sort/radixsort.go#L35): No description provided.
+14. [`SelectionSort`](./sort/selectionsort.go#L3): No description provided.
+15. [`ShellSort`](./sort/shellsort.go#L3): No description provided.
+16. [`SimpleSort`](./sort/simplesort.go#L11): No description provided.
 
 ---
 ##### Types

diff --git a/math/kthnumber.go b/math/kthnumber.go
@@ -0,0 +1,43 @@
+package math
+
+import (
+	"github.com/TheAlgorithms/Go/search"
+	"github.com/TheAlgorithms/Go/sort"
+)
+
+// FindKthMax returns the kth large element given an integer slice
+// with nil `error` if found and returns -1 with `error` `search.ErrNotFound`
+// if not found. NOTE: The `nums` slice gets mutated in the process.
+func FindKthMax(nums []int, k int) (int, error) {
+	index := len(nums) - k
+	return kthNumber(nums, index)
+}
+
+// FindKthMin returns kth small element given an integer slice
+// with nil `error` if found and returns -1 with `error` `search.ErrNotFound`
+// if not found. NOTE: The `nums` slice gets mutated in the process.
+func FindKthMin(nums []int, k int) (int, error) {
+	index := k - 1
+	return kthNumber(nums, index)
+}
+
+// kthNumber use the selection algorithm (based on the partition method - the same one as used in quicksort).
+func kthNumber(nums []int, k int) (int, error) {
+	if k < 0 || k >= len(nums) {
+		return -1, search.ErrNotFound
+	}
+	start := 0
+	end := len(nums) - 1
+	for start <= end {
+		pivot := sort.Partition(nums, start, end)
+		if k == pivot {
+			return nums[pivot], nil
+		}
+		if k > pivot {
+			start = pivot + 1
+			continue
+		}
+		end = pivot - 1
+	}
+	return -1, search.ErrNotFound
+}
diff --git a/math/kthnumber_test.go b/math/kthnumber_test.go
@@ -0,0 +1,104 @@
+package math
+
+import (
+	"github.com/TheAlgorithms/Go/search"
+	"testing"
+)
+
+func TestFindKthMax(t *testing.T) {
+	sortTests := []struct {
+		input    []int
+		k        int
+		expected int
+		err      error
+		name     string
+	}{
+		{
+			input:    []int{6, 7, 0, -1, 10, 70, 8, 22, 3, 9},
+			k:        3,
+			expected: 10,
+			name:     "3th largest number",
+		},
+		{
+			input:    []int{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1},
+			k:        3,
+			expected: -1,
+			name:     "3th largest number",
+		},
+		{
+			input:    []int{-1, -1, -1, -1, -1, -1},
+			k:        7,
+			expected: -1,
+			err:      search.ErrNotFound,
+			name:     "This should be an error",
+		},
+		{
+			input:    []int{},
+			k:        1,
+			expected: -1,
+			err:      search.ErrNotFound,
+			name:     "This should be an error",
+		},
+	}
+	for _, test := range sortTests {
+		t.Run(test.name, func(t *testing.T) {
+			actual, err := FindKthMax(test.input, test.k)
+			if err != test.err {
+				t.Errorf("name:%v FindKthMax() = %v, want err: %v", test.name, err, test.err)
+			}
+			if actual != test.expected {
+				t.Errorf("test %s failed", test.name)
+				t.Errorf("actual %v expected %v", actual, test.expected)
+			}
+		})
+	}
+}
+
+func TestFindKthMin(t *testing.T) {
+	sortTests := []struct {
+		input    []int
+		k        int
+		expected int
+		err      error
+		name     string
+	}{
+		{
+			input:    []int{6, 7, 0, -1, 10, 70, 8, 22, 3, 9},
+			k:        3,
+			expected: 3,
+			name:     "3th smallest number",
+		},
+		{
+			input:    []int{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1},
+			k:        3,
+			expected: -1,
+			name:     "3th smallest number",
+		},
+		{
+			input:    []int{-1, -1, -1, -1, -1, -1},
+			k:        7,
+			expected: -1,
+			err:      search.ErrNotFound,
+			name:     "This should be an error",
+		},
+		{
+			input:    []int{},
+			k:        1,
+			expected: -1,
+			err:      search.ErrNotFound,
+			name:     "This should be an error",
+		},
+	}
+	for _, test := range sortTests {
+		t.Run(test.name, func(t *testing.T) {
+			actual, err := FindKthMin(test.input, test.k)
+			if err != test.err {
+				t.Errorf("name:%v FindKthMin() = %v, want err: %v", test.name, err, test.err)
+			}
+			if actual != test.expected {
+				t.Errorf("test %s failed", test.name)
+				t.Errorf("actual %v expected %v", actual, test.expected)
+			}
+		})
+	}
+}
diff --git a/sort/quicksort.go b/sort/quicksort.go
@@ -7,7 +7,7 @@
 
 package sort
 
-func partition(arr []int, low, high int) int {
+func Partition(arr []int, low, high int) int {
 	index := low - 1
 	pivotElement := arr[high]
 	for i := low; i < high; i++ {
@@ -27,7 +27,7 @@ func QuickSortRange(arr []int, low, high int) {
 	}
 
 	if low < high {
-		pivot := partition(arr, low, high)
+		pivot := Partition(arr, low, high)
 		QuickSortRange(arr, low, pivot-1)
 		QuickSortRange(arr, pivot+1, high)
 	}