LeetCode – Two Sum (Java)

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

For example:

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

Naive Approach

This problem is pretty straightforward. We can simply examine every possible pair of numbers in this integer array.

Time complexity in worst case: O(n^2).

public static int[] twoSum(int[] numbers, int target) {
	int[] ret = new int[2];
	for (int i = 0; i < numbers.length; i++) {
		for (int j = i + 1; j < numbers.length; j++) {
			if (numbers[i] + numbers[j] == target) {
				ret[0] = i + 1;
				ret[1] = j + 1;
			}
		}
	}
	return ret;
}

Can we do better?

Better Solution

Use HashMap to store the target value.

public class Solution {
    public int[] twoSum(int[] numbers, int target) {
	HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
	int[] result = new int[2];
 
	for (int i = 0; i < numbers.length; i++) {
		if (map.containsKey(numbers[i])) {
			int index = map.get(numbers[i]);
			result[0] = index+1 ;
			result[1] = i+1;
			break;
		} else {
			map.put(target - numbers[i], i);
		}
	}
 
	return result;
    }
}

Time complexity depends on the put and get operations of HashMap which is normally O(1).

Time complexity of this solution: O(n).