CodeSignal-Arrays(1):firstDuplicate

Question

Given an array a that contains only numbers in the range from 1 to a.length, find the first duplicate number for which the second occurrence has the minimal index. In other words, if there are more than 1 duplicated numbers, return the number for which the second occurrence has a smaller index than the second occurrence of the other number does. If there are no such elements, return -1.

Example

• For a = [2, 1, 3, 5, 3, 2], the output should be solution(a) = 3.

  There are 2 duplicates: numbers 2 and 3. The second occurrence of 3 has a smaller index than the second occurrence of 2 does, so the answer is 3.

• For a = [2, 2], the output should be solution(a) = 2;

• For a = [2, 4, 3, 5, 1], the output should be solution(a) = -1.

Input/Output

• [execution time limit] 3 seconds (java)

• [memory limit] 1 GB

• [input] array.integer a

  Guaranteed constraints:
  1 ≤ a.length ≤ 105,
  1 ≤ a[i] ≤ a.length.

• [output] integer

  The element in a that occurs in the array more than once and has the minimal index for its second occurrence. If there are no such elements, return -1.

Answer java

int solution(int[] array) {
   if (array == null || array.length < 2) return -1;
		for (int val : array) {
			int absVal = Math.abs(val);
			if (array[absVal - 1] < 0) return absVal;
			array[absVal - 1] *= -1;
		}
    return -1;

}