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