1.Two Sum

题目：
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

个人理解：找出和等于目标整数的两个数的位置，位置不是从0开始（那就是从1开始咯）。双重for循环效率很低，所以使用HashMap辅助解决快很多。

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