*[Lintcode]Two Sum 两数之和

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.

Example

numbers=[2, 7, 11, 15], target=9

return [1, 2]

分析：题目要求复杂度：

• O(n) Space, O(nlogn) Time
• O(n) Space, O(n) Time

明显要使用额外空间。这里使用hashmap储存key=还差多少等于target value=index

public class Solution {
    /*
     * @param numbers : An array of Integer
     * @param target : target = numbers[index1] + numbers[index2]
     * @return : [index1 + 1, index2 + 1] (index1 < index2)
     */
    public int[] twoSum(int[] numbers, int target) {
        HashMap<Integer, Integer> res = new HashMap<Integer, Integer>();
        
        for(int i = 0; i < numbers.length; i++) {
            if(res.containsKey(numbers[i])) {
                return new int[]{res.get(numbers[i]) + 1, i + 1};
                
            } else {
                res.put(target - numbers[i], i);
            }
        }
        return new int[]{};
    }
}