LEETCODE 0001 TWO SUM

1. Two Sum

Difficulty	Topics	Hints	URL
Easy	Array``Hash Table	3	https://leetcode.com/problems/two-sum/description/

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Table of Contents

• Description
• Examples
• Solutions

Description

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Examples

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

Constraints:

• 2 <= nums.length <= 104
• -109 <= nums[i] <= 109
• -109 <= target <= 109
• Only one valid answer exists.

Follow-up: Can you come up with an algorithm that is less than O(n2) time complexity?

Solutions

JAVA (Array)

class Solution {
    public int[] twoSum(int[] nums, int target) {
                int numsLength = nums.length;
        for (int i = 0; i < numsLength - 1; ++i) {
            for (int j = i + 1; j < numsLength; ++j) {
                if (target == nums[i] + nums[j]) {
                    return new int[]{i, j};
                }
            }
        }
        return new int[2];
    }
}

JAVA (Hash Table)

import java.util.HashMap;

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];

            if (map.containsKey(complement)) {
                return new int[] { map.get(complement), i };
            }
            map.put(nums[i], i);
        }
        return new int[0];
    }
}