1. 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, and you may not use the same element twice.

Example:

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

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

Solution

(1) Java

class Solution {
    public int[] twoSum(int[] nums, int target) {
        if (nums == null || nums.length == 0) {
            return new int[]{-1,-1};
        }
        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};
            }
            map.put(nums[i], i);
        }
        return new int[]{-1,-1};
    }
}

(2) Python

class Solution:
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        if not nums:
            return []
        cache = {}
        for i, num in enumerate(nums):
            if target-num in cache:
                return [cache[target-num], i]
            cache[num] = i
        return []

(3) Scala

object Solution {
    def twoSum(nums: Array[Int], target: Int): Array[Int] = {
        nums.zipWithIndex.filter(x =>
        (nums.take(nums.indexOf(x._1)) ++ nums.drop(nums.indexOf(x._1)+1))
        .contains(target - x._1)).map(_._2)
    }
}