Two Sum

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.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Output: 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]

Approach-1 :- Brute Force__ Time: O(n²) & Space: O(1)

public int[] twoSum(int[] nums, int target) {
    for (int i = 0; i < nums.length; i++) {
        for (int j = i + 1; j < nums.length; j++) {
            if (nums[j] == target - nums[i]) {
                return new int[] { i, j };
            }
        }
    }
    throw new IllegalArgumentException("Not Possible");
}

• Time complexity : O(n²), For each element, we try to find its complement by looping through the rest of array which takes O(n) time. Therefore, the time complexity is O(n²).
• Space complexity : O(1).

Approach-2 :- Map__ Time: O(n) & Space: O(n)

public int[] twoSum(int[] nums, int target) {
    Map<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);
    }
    throw new IllegalArgumentException("Not Found");
}

• Time complexity : O(n). We traverse the list containing n elements only once.
• map.containsKey(key) — takes O(1) time.
• Space complexity : O(n). The extra space required depends on the number of items stored in the hash table, which stores at most n elements.