Question:

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.

You may assume that each input would have exactly one solution.

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

Thinking:

We can use hash map to store the rest of nums[i], and if we find which number is equal to the rest beofore, their sum will be target.

Solution:

public int[] twoSum(int[] nums, int target){
    HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    int[] res = new int[2];

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

    return res;
}