Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example: Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].

Solution)

To use a hashtable to remember the index of the second number to make a target.

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Set<Integer> table = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (table.get(nums[i]) != null) {
                return new int[] {table.get(nums[i]), i};
            } else {
                table.put(target-nums[i], i);
            }
        }
        return new int[] {-1, -1};
    }
}

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(target-nums[i])) return new int[]{map.get(target-nums[i]), i};
            else map.put(nums[i], i);
        }
        return null;
    }
}