leetcode-- two sum

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

answer:
attention that we should take O() into consideration

java:

• this main idea take advantage of hashtable,cause when you find some number in the table ,only cost O(1) .
• find at the same time fill the hashtable