*[Lintcode]Two Sum 兩數之和

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：題目要求複雜度：

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

明顯要使用額外空間。這裏使用hashmap儲存key=還差多少等於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[]{};
    }
}