Given an array of integers, return indices of the two numbers such that they add up to a specific target.

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

使用HashMap存储数字，对每一个元素，查找target-nums[i]元素对应的下标，如果找到则返回结果。

java code:

public class Solution {
  public int[] twoSum(int[] nums, int target) {
    HashMap map = new HashMap<Integer, Integer>();
    int[] result = new int[2];
    
    for (int i = 0; i < nums.length; i++) {
      Integer number = (Integer)map.get(nums[i]);
      if (number == null) map.put(nums[i], i);
        
      number = (Integer) map.get(target - nums[i]);
        
      if (number != null && number != i) {
        result[0] = number;
        result[1] = i;
        return result;
      }  
    }
    return result;
  }
}