1. Two Sum

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> valueToIndexMap = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            Integer anotherIndex = valueToIndexMap.get(target - nums[i]);
            if (anotherIndex != null) {
                return new int[]{i, anotherIndex};
            } else {
                valueToIndexMap.put(nums[i], i);
            }
        }
        throw new RuntimeException("Cannot find target sum in nums");
    }
}

Reference

1. Two Sum