Two Sum - Leetcode #1

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Question

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.

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]

Link: https://leetcode.com/problems/two-sum/

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Intuition

• Brute Force - search for any two elements in two separate iterations to find any two numbers that add up to a given target.

  TC: O(n²)

  SC: O(1)

• Optimized Solutions #1 - Two-pass Hashmap

  • Place each value in the hashmap and its corresponding array element as a key in the first iteration, and on the second iteration, check if there exists any key such that key = target - arr[i]. If so, a solution has been found.

    TC: O(n)

    SC: O(n)

• Optimized Solutions #2 - Two-pass Hashmap

  • Same as #1, just with us checking if there exists a value within the hashmap that satisfies the condition before pushing the aforementioned key into the hashmap.

    TC: O(n)

    SC: O(n)

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Solution #1

class Solution { // double pass
    public int[] twoSum(int[] nums, int target) {
        int arr[] = new int[2];
        HashMap<Integer, Integer> hmap = new HashMap<Integer, Integer>();

        for(int i=0; i<nums.length; i++) {
            hmap.put(nums[i],i);
        }
        for(int i=0; i<nums.length; i++) {
            if(hmap.containsKey(target-nums[i])) {
                arr[0] = hmap.get(target-nums[i]);
                arr[1] = i;
                break;
            }
        }
        return arr;
    }
}

Solution #2

class Solution { // single pass
    public int[] twoSum(int[] nums, int target) {
        int arr[] = new int[2];
        HashMap<Integer, Integer> hmap = new HashMap<Integer, Integer>();

        for(int i=0; i<nums.length; i++) {
            if(hmap.containsKey(target-nums[i])) {
                arr[0] = hmap.get(target-nums[i]);
                arr[1] = i;
                break;
            }
            hmap.put(nums[i],i);
        }
        return arr;
    }
}

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Links:

DSA Prep: https://github.com/Dijkstra-Edu/DSA-Fundamentals---JAVA

Leetcode Solutions: https://github.com/Dijkstra-Edu/LeetCode-Solutions

Root Repository: https://github.com/Dijkstra-Edu/DSA-Handbook