LeetCode 1. Two Sum（两数之和）

原题网址：https://leetcode.com/problems/two-sum/

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

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

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

UPDATE (2016/2/13):
The return format had been changed to zero-based indices. Please read the above updated description carefully.

方法：使用Map保存数值的位置。

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for(int i=0; i<nums.length; i++) {
            Integer pos = map.get(target-nums[i]);
            if (pos != null) {
                int[] pair = new int[2];
                pair[0] = pos;
                pair[1] = i;
                return pair;
            }
            map.put(nums[i], i);
        }
        return null;
    }
}