add: original O(logn) solution for challenge3rotationcount

Pattern11 - Modified Binary Search/Challenge3-Rotation_Count/solution.go

@@ -0,0 +1,47 @@
+package challenge3rotationcount
+
+/*
+Given an array of numbers which is sorted in ascending order and is rotated ‘k’ times around a pivot, find ‘k’.
+
+You can assume that the array does not have any duplicates.
+
+Input: [10, 15, 1, 3, 8]
+Output: 2
+Explanation: The array has been rotated 2 times.
+
+Input: [4, 5, 7, 9, 10, -1, 2]
+Output: 5
+Explanation: The array has been rotated 5 times.
+
+Input: [1, 3, 8, 10]
+Output: 0
+Explanation: The array has been not been rotated.
+
+ref: https://leetcode-cn.com/problems/find-minimum-in-rotated-sorted-array/
+*/
+
+func countRotations(arr []int) int {
+	var (
+		start = 0
+		end   = len(arr) - 1
+	)
+
+	for start < end {
+		mid := start + (end-start)/2
+		if mid < end && arr[mid] > arr[mid+1] {
+			return mid + 1
+		}
+		if mid > start && arr[mid-1] > arr[mid] {
+			return mid
+		}
+
+		if arr[start] < arr[mid] {
+			// left part sorted, so pivot is right part
+			start = mid + 1
+		} else {
+			end = mid - 1
+		}
+	}
+
+	return 0
+}

Pattern11 - Modified Binary Search/Challenge3-Rotation_Count/solution_test.go

@@ -0,0 +1,26 @@
+package challenge3rotationcount
+
+import "testing"
+
+func Test_countRotations(t *testing.T) {
+	type args struct {
+		arr []int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{"case1", args{[]int{10, 15, 1, 3, 8}}, 2},
+		{"case2", args{[]int{4, 5, 7, 9, 10, -1, 2}}, 5},
+		{"case3", args{[]int{1, 3, 8, 10}}, 0},
+		{"case4", args{[]int{10, 1, 3, 8, 9}}, 1},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := countRotations(tt.args.arr); got != tt.want {
+				t.Errorf("countRotations() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}