Given a list of integers and a target sum, return the indices of the two numbers that add up to the target.

You may assume that each input has exactly one solution, and you cannot use the same element twice. Return the indices as a list of two integers [index1, index2] where index1 < index2.

Method Signature:

public static List<Integer> twoSum(List<Integer> nums, Integer target)

Examples:

Example 1:

• Input: nums = [2, 7, 11, 15], target = 9
• Output: [0, 1]
• Explanation: nums[0] + nums[1] = 2 + 7 = 9, so return [0, 1]

Example 2:

• Input: nums = [3, 2, 4], target = 6
• Output: [1, 2]
• Explanation: nums[1] + nums[2] = 2 + 4 = 6, so return [1, 2]

Example 3:

• Input: nums = [3, 3], target = 6
• Output: [0, 1]
• Explanation: Both elements at index 0 and 1 add up to 6

Example 4:

• Input: nums = null, target = 9
• Output: null
• Explanation: Return null for null input