题目描述:

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

给一个整型数组，找出数组中两个值，使得这两个值之和等于target，并返回下标，下标1必须小于下标2.

您可以假设每个输入都只有一个解决方案。

解题思路：

使用HashMap<target-numbers[i], i>，从头遍历数组，先判断是否存在key等于numbers[i]，如果存在，则返回索引，不存在则执行map.put(target-numbers[i], i);

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