LeetCode 1. Two Sum

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]

Constraints:

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

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

Solution

public int[] twoSum(int[] nums, int target) {
    // nums[i] -> i
    Map<Integer, Integer> map = new HashMap<>();
    // fix the order within each pair as (left, right), so that avoid duplicates
    for (int right = 0; right < nums.length; right++) {
        int n = nums[right];
        if (map.containsKey(target - n)) {
            return new int[]{map.get(target - n), right};
        } else {
            map.put(n, right);
        }
    }
    // cannot find such pair
    return null;
}