Two Sum

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

//想法：遍历数组的同时，检查hashmap中是否存在target - number[i]， 如果存在，则返回target-number[i]在数组中的index+2，若不存在，就将number[i]存放到hashmap里面。 因为hashmap一开始为空，所以一开始会把数组中的第一个值存到hashmap中.按照这种顺序，hashmap中的值在数组中的index一定是小于当前在数组中遍历到的值,所以index1 = numberHash.get(tar - numbers[i]);
int[] twoSum(int[] numbers, int target){
    if(numbers.length < 2 || numbers == null){
        return null;
    }
    HashMap<Integer,Integer> numbersHash = new HashMap<Integer,Integer>(); 
    for(int i = 0; i < numbers.length; i++){
        if(numbersHash.containsKey(target - numbers[i])){
            int index1 = numbersHash.get(target - numbers[i]) + 1;
            int index2 = i + 1;
            int[] index = {index1, index2};
            return index;
        }else{
            numbersHash.put(numbers[i],i);
        }
    }
    return null;
}