improve find duplicates problem description and tests (#142)

Author: spring1843

array/find_duplicate_in_array.go

@@ -1,11 +1,13 @@
 package array
 
+import "math"
+
 // FindDuplicate solves the problem in O(n) time and O(1) space.
 func FindDuplicate(list []int) int {
 	for _, item := range list {
-		itemIndex := item - 1
+		itemIndex := int(math.Abs(float64(item))) - 1
 		if list[itemIndex] < 0 {
-			return list[itemIndex] * -1
+			return item
 		}
 		list[itemIndex] *= -1
 	}

array/find_duplicate_in_array_test.go

@@ -7,7 +7,8 @@ TestFindDuplicate tests solution(s) with the following signature and problem des
 
 	FindDuplicate(list []int) int
 
-Given a list of integers (1,2,...,n), find a duplicate number in O(n) time.
+Given a list of n integers (3, 2, 1, 4, 5, 4,...,n) where each number is positive and smaller
+than n find the duplicate integer.
 */
 func TestFindDuplicate(t *testing.T) {
 	tests := []struct {
@@ -19,6 +20,8 @@ func TestFindDuplicate(t *testing.T) {
 		{[]int{1, 2, 3}, -1},
 		{[]int{1, 1, 2, 3}, 1},
 		{[]int{1, 2, 2, 3}, 2},
+		{[]int{1, 2, 3, 2, 4, 5}, 2},
+		{[]int{3, 2, 1, 4, 5, 4}, 4},
 	}
 
 	for i, test := range tests {