[LintCode] 56 Two Sum 解题报告

Description
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.


Notice
You may assume that each input would have exactly one solution



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

return [1, 2]



Challenge
Either of the following solutions are acceptable:

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



思路
过一遍数组，并在HashMap里找有没有target - 当前元素。如果有就找到了。如果没有，就把当前元素以及index放进HashMap。



Code

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) {
        // write your code here
        
        HashMap<Integer, Integer> map = new HashMap<>();
        
        for (int i = 0; i < numbers.length; i++) {
            if (map.containsKey(target - numbers[i])) {
                return new int[] {map.get(target - numbers[i]), i + 1};
            }
            map.put(numbers[i], i + 1);
        }
        
        return null;
    }
}